The purpose of this doc is to collect the most common terms/considerations that come up while designing features for JavaScript at TC39. It's a work in progress and needs your help to grow! See below for instructions as well as terms/considerations in need of definition.
These are common terms used while discussing language features.
The process of discussing a trivial matter at the expense of the topic that actually needs discussion. This can take time away from important topics, and its important to catch ourselves if we start bikeshedding!
We were supposed to discuss how this new proposal should work, but we spent the entire time discussing what the name should be. We should avoid such bikeshedding.
Brand check ("brand" as in a mark, or a brand made with a branding iron) is a term used by TC39 to describe a check against a unique datatype whose creation is controlled by a piece of code.
One example of this is built in JavaScript datatypes, which are unique and cannot be made in user
space. Array.isArray is an example of a brand check. For reference see this
discussion.
A common misconception is that instanceof is a brand check. This is a nominal type check and does
not reliably determine the type. It used to be that a brand check was only possible for built in
types. For a more detailed explanation, see this write
up.
It is now possible to implement brand checks in user space as long as there is a way to identify that the object is unique.
Imagine a library that implements DOM queries and returns a query object. The author of this
library may be interested in being able to modify the implementation of the query object without
breaking the programs of users of the library. However, returning plain objects such as { type: "queryResult", elements: [ ...... ] } is not safe, as anyone can return such an object and create a
forgery of a query object. In order to avoid this, the library must make a brand check to ensure
that this object indeed belongs to the library. That can be done like so:
const queries = new WeakMap();
class Query {
// ...
performQuery(queryString) {
// returns the query object
return { type: "queryResult", elements: [ ...... ] };
}
get query(query) {
queries.get(query); // verifies that the query exists as a member of the WeakMap
}
set query(queryString) {
// generate a query object
const query = this.performQuery(queryString);
// use the object itself as the key
queries.set(query, ...);
}
}A tail call is a call which occurs as the final operation of a function and whose value is returned immediately. It is possible for such a call to reuse or replace the current stack frame, in which case it is known as a proper tail call (PTC). PTC semantics are part of the standard as of ES6, but their implementation in various engines has been fraught with controversy. In particular, reluctance to the automatic nature of PTC led to an alternative syntactic tail call (STC) proposal, in which users would consciously choose this behavior by means of a keyword. PTC is currently only shipped by JSC, while STC remains an open but inactive proposal.
function factorial(n) {
// Not a tail call -- we still need to multiply by n after the call
return (n <= 1) ? 1 : n * factorial(n - 1);
}
function factorial(n, acc = 1) {
// Tail call -- achieved by passing an accumulator argument
return (n <= 1) ? acc : factorial(n - 1, n * acc);
}
function factorial(n, acc = 1) {
// Tail call with opt-in stack frame elision (STC example)
return (n <= 1) ? acc : continue factorial(n - 1, n * acc);
}Refers to a period of time during which a variable has been declared, but has
not been assigned, and is therefore unavailable. This results in a ReferenceError. This happens when
a const or a let is defined in the scope, but not yet. This is different from var, which will
return undefined. Here is an example:
console.log(bar) // ReferenceError TDZ
console.log(baz) // undefined
let bar = 1;
var baz = 2;
console.log(bar) // 1
console.log(baz) // 2- Let and Const Declarations -- the ECMAScript specification
- Temporal Dead Zone -- blog post by Wes Bos which describes the term
A "cover grammar" is a technique used in the JavaScript grammar to remain context free and unambiguous when parsing from left to right with only one token of lookahead, while later tokens might lead to syntactic restrictions for earlier ones. Informally, one grammar is said to "cover" another if the second grammar is a subset of the first, with a corresponding subset of corresponding parse trees.
The CoverParenthesizedExpressionAndArrowParameterList production in the ECMAScript specification allows expressions and destructuring parameters of arrow functions to be interpreted together. If a => is reached, the expression is reinterpreted as arrow function parameters, with additional restrictions applied.
The definition of covering in the ECMAScript specification, used to check whether it's valid to reinterpret one grammatical production as another.
A change to JavaScript is considered "web compatible" if it preserves the current behavior of existing websites. If it changes the behavior of existing websites, it's considered to "break the web".
The definition here is a bit fuzzy and empirical--it's always possible to construct a website which will break under any particular change or addition to JavaScript, and the key is how common the broken websites are. If too many websites break, then web browsers will refuse to ship the change.
"Don't break the web" is a shared goal of TC39 and all web standards bodies: We aim to preserve web compatibility as we evolve the language. Even if a change would be convenient for developers, it's not worth it if we hurt lots of users in the process!
There was an effort to add a method Array.prototype.contains. However, this broke many websites (reference). As a result, the method was named as Array.prototype.includes instead.
ES2015 changed RegExp semantics to make RegExp.prototype not a RegExp instance, and to make RegExp.prototype.sticky a getter which threw when accessed on a non-RegExp. It turned out that this change was not web-compatible--data from Chrome showed that .05% of web page loads hit this case.
As a result, TC39 agreed to make an allowance in RegExp.prototype.sticky and similar getters to permit RegExp.prototype as a receiver. See this slide deck for details.
- #SmooshGate FAQ by Mathias Bynens
"Meta-object protocol" (often abbreviated as "MOP") is a fancy term to describe the basic operations in an object system. For example, in JavaScript, getting a property is one operation in the meta-object protocol. The term originated in the Lisp community, and is used in TC39 because we can take a lot of inspiration from the developments in the Common Lisp Object System.
Each operation in JavaScript's meta-object protocol is a method in Reflect, and each of these is also a Proxy trap.
In the issue Implement meta-object trap(s) to make an object's [[Prototype]] immutable, there is discussion about adding another fundamental object operation to freeze the prototype of an object (without performing a full preventExtensions).
The title of the issue includes "meta-object" as a reference to the meta-object protocol, as the addition of this feature would require new Proxy and Reflect APIs.
- The Art of the Metaobject Protocol -- the book which introduced the term
- Object Internal Methods and Internal Slots -- JavaScript's meta-object protocol
An "early error" is an error which is thrown in the parsing phase of JavaScript code, before executing. This error is usually a SyntaxError, but other error types may be early errors as well. These early errors are produced even if the relevant code is not executed. Early errors are contrasted with runtime errors, which happen in the course of executing JavaScript code.
When a JavaScript Script, Module or eval'd string contains an early error, none of it is run.
Multiple declarations of the same lexical variable produces an early error. For example, the following produces an early SyntaxError.
const x = 1;
const x = 2;By contrast, referencing a variable which is not defined is a runtime error, specifically a ReferenceError.
let abc = 1;
console.log(abd); // runtime error- early error defined in the ECMAScript specification editor's draft.
A parameter value that is a JavaScript object with properties looked up eagerly. It is used to configure behavior of a function. Methods on the options bag object should be invoked with undefined as the default value for this
The definition here is a bit fuzzy as some parameters are objects but not options bags.
// options is an options bag
const options = { style: 'currency', currency: 'EUR' };
new Intl.NumberFormat('de-DE', options);"Memoization" is an optimization technique to reduce the run time of a program by using a cache to store results and avoid recomputation. In essence, it is a trade-off of space in exchange for time.
Non-memoized Fibonacci
function fib(num) {
if (num <= 1) return 1;
return fib(num - 1) + fib(num - 2);
}Memoized Fibonacci
function memoizedFib(num, memo = {}) {
if (memo[num]) return memo[num];
if (num <= 1) return 1;
return memo[num] = memoizedFib(num - 1, memo) + memoizedFib(num - 2, memo);
}A read-eval-print loop, i.e. "REPL", is a interactive programming environment that allows a user to input a single expression whose result will be evaulated and returned. After each read-eval-print the environment returns to the initial read state, creating a loop, which is only terminated once the environment is closed.
In Firefox's Web Console the command line interpreter is a JavaScript REPL.
An agent is a comprised of three things: a container for a set of job queues, related execution state, and an execution thread. The execution state is exclusive to the agent while the execution thread can be shared with other agents.
A realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources (i.e. a global object and an associated set of primordial objects). Today, in the browser, realms can be created via same origin iframes.
A built-in value that is required by the ECMA262 specification. Where observable (e.g., as primordial), intrinsic objects are realm-specific while intrinsic symbols are shared by all realms. The specification itself references "well-known" intrinsics with special notation (%<name>% for objects; @@<name> for symbols).
%ForInIteratorPrototype% is the prototype of internal iterators that can be used to implement for ( Identifier in Expression ) statements
ECMA262 (objects), ECMA262 (symbols)
An intrinsic value that is accessible to ECMAScript code and required to exist before any ECMAScript code runs.
%Array%, the initial value of a realm's Array constructor, is accessible as Array.
A primordial value that is accessible to ECMAScript code without reference to named bindings other than those for prototype and property descriptor reflection (i.e., solely by syntax, __proto__, and primordial getPrototypeOf/getOwnPropertyDescriptor/getOwnPropertyDescriptors functions).
%Array.prototype%, the initial value of a realm's Array prototype, is accessible as [].__proto__.
%ThrowTypeError%, a realm's special TypeError-throwing function, is accessible as (function(){ 'use strict'; return Object.getOwnPropertyDescriptor(arguments, 'callee').get; })().
Statements that constrain the observable behavior of implementations (i.e., the behavior that conforming implementations are allowed to exhibit).
Secure ECMAScript. A subset of ECMAScript.
Meeting of all available TC39 delegates. Occurs around 6 times a year.
A seemingly simple change (i.e. thought to be semantically equivalent) that seems like a drop in replacement for an existing piece of code but has specific edge cases that make it not work as expected. As you refactor code from "old pattern" to "new pattern", it would be easy to cause unintentional effects (bugs, etc).
To make something a first-class concept in a language specification, such that it can be passed around, inspected, or manipulated by user code.
- The C programming language reifies the concept of memory addresses with pointers.
- ECMAScript reifies its own interpreter with
eval.
SDOs are syntax-directed operations in the ECMA-262 specification. These are the operations that are defined piecewise over the parse tree rather than by having a single algorithm. In other terms it is a particular style of algorithm in the specification.
Evaluation is an SDO because Evaluation is defined piecewise over several productions.
SDO is short for a Standards Developing Organization.
- ECMA
- Unicode Consortium
The idea that there's an upper bound on how much syntactic complexity a language can reasonably contain, expressed as a metaphorical budget.
A proposal without syntactic implications, such as the addition of a new method to a built-in, may be relatively uncontroversial, but a proposal that introduces new syntax is expected to justify its cost against this budget. This means demonstrating both (1) that it addresses a significant developer need in a way that would not be reasonably achieved without new syntax and (2) that the specific syntax being proposed does not obstruct another proposal expected to advance in the foreseeable future.
Additionally, in a shorter-term sense, it could overwhelm developers to have too many syntax-heavy features ship in too narrow of a timeframe.
Used mainly in opposition to Out of band, denoting that all configuration of a feature or a behavior is done within the context of ECMASCript source.
- The stage 3 Import assertions proposal assert pragma
- The "use strict" directive, an in-band way to enable strict mode for some portion of code.
Import assertions proposal FAQ in opposition to out-of-band
Used to denote a behavior that applies to the execution of a ECMAScript module or script that is configured outside of ECMAScript source text.
An example of out-of-band configuration is Import Maps. With this feature, the module specifier used in an import statement or the dynamic import function is interpreted by the host differently based on the import map configuration that is defined within a non-ECMAScript source. This may be a HTML <script> tag with the importmap type in the browser, or in a configuration file like package.json in a Node.js project.
Another example is whether an ECMAScript source file will be executed as a script or a module based on the type attribute of the <script> tag that caused the User Agent to fetch and load it within a browser context. The same is done in Node.js through the use of the .mjs extension or by specifying that the package is of type "module" in the pacakge.json file.
Override mistake is the behavior that ECMAScript throws a TypeError (in the strict mode) for code O[K] = ... when K is a non-writable property in the prototype of O.
This behavior brings compatibility issues in the ecosystem when any properties are non-writable, including when the language adds new built-in non-writable properties to existing prototypes and when the runtime or code on the page freezes built-ins.
'use strict';
// environment
Object.freeze(Error.prototype);
// an es5 library
function MyError() {
Error.call(this);
this.name = 'MyError';
}
Object.setPrototypeOf(MyError.prototype, Error.prototype);
// TypeError: name is readonly
new MyError;Prior efforts trying to change this: Normative: Make non-writable prototype properties not prevent assigning to instance
Some real-world examples:
- tc39/proposal-iterator-helpers: Web compat: The %Symbol.toStringTag% property on %Iterator.prototype% needs to be writable
- tc39/proposal-iterator-helpers: Web incompatibility discovered in theathletic.com
- Tracking issue for getting 3rd party packages more SES friendly (some of the reports are related)
The idea that when differing perspectives come into conflict, our decisions ought to prioritize them in the following (descending) order:
- End users (tool and website users)
- JavaScript authors
- JavaScript engine implementers
- ECMAScript specification authors
- Theoretical purity
This ordering is not absolute; a small cost or benefit to a higher-priority group can be overridden by a sufficiently large cost or benefit to a lower-priority group.
While this is not an explicitly adopted goal of TC39, it is a common standards concept which delegates often refer to.
Here are some tips and ideas for adding a new definition to this document.
When you add a definition, make sure that the definition applies to how TC39 uses it. Some other communities might have similar terms, but they mean a different thing in this case. Otherwise, feel free to reference well known definitions so that people know what they mean.
Anatomy of a good definition:
- in simple words, what is it? Imagine describing it to someone who has no experience
- a minimal example
- sources and resources where people can learn more
- related definitions (optional)
## Term
### Definition
definition of term
### Example
example of term
### References
referencesThese are terms which have been previously identified as worthy of defining.
- hoisting
- IIFE
- hyrum’s law
- POLA: Principle Of Least Authority
- Sigil
- Contextual keywords
- parsing ambiguity
- readability ambiguity
- lexical scope
- Usability/Ergonomics
- Cornering
- Consistency
- Coherency
- Cross cutting concerns
- 1JS position
- Cornering
- Frogboiled
- Footguns
- Mangling
- EIBTI: explicit is better than implicit
- IIFE: immediately invoked function expression
- Hoisting
- Enumerability
- Observable
- 3 Passes: (a) parsing, (b) go over it ahead of time for “early errors”, (c) execute (runtime errors)
- Proxies/membranes (mark miller, @tvcutsem),
- Currying operation -- wycats
- orthogonal
- "Needs Consensus PR"
- web reality
- (meeting) minutes
- editor, editor group, project editors
- Test262
- ECMA 262, 402, 404
- language semantics, grammar
- Stages, Stage 0-4
- Reflector
- Proposal
- Disjointed proposals
- Host
- Engines: v8, SpiderMonkey, Chakra, JavaScriptCore
- Timebox
- Spec/Spec text
- cargo-cult
- One JS
- lazy parsing
- self-hosted
- overhead
These are common considerations that come up while discussing the technical merits of language features.
- Syntax constraints (waldemar)
- Parsing costs (backtracking, multiple parsing trees before getting tokens, e.g. pipeline
- ASI hazards, semicolons hazards
- Tokenization is greedy -- waldemar
- Punctuation overload -- mark miller
- Does it belong to the language? The extensible web manifesto.
- Complexity Budget (herman, allen)
- The cost of growing a language, the role of polyfils (adamk)
- Does it stand on its own weight? Does it pay for itself? (mark miller)
- The principle of least surprise
- Path dependence
- Readability and writability: more people read than write
- Forward compatibility
- Tennent’s Correspondence Principle (dherman)
- Backwards compatibility (domenic, jordan)
- object capabilities
- Single origin policy
- communication channel
- tampering
- Popular opinion (jordan, taking the feedback from the community constructively)
- Polyfill considerations (libraries, frameworks, etc)
- Transpilers (typescript, flow, coffeescript, babel)
- Frameworks (angular, polymer, react, etc)
- Implementation considerations (Vv8)
- Other Standards (@jordan, overlap of HTML and now WASM, coordinating between standards communities, e.g. should standard libraries go to WHATWG or should them be JS, where do we draw the line?, tie the hands of TC39 when others move first, and vice versa)
- Fragmentation, convergence and the role of Standards
- Die on that hill