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JavaScript is the familiar name for a programming language whose formal definition is published as ECMAScript. The ECMA-262 standard specifies the language’s syntax, behavior, built-in values and objects; environments such as browsers provide additional facilities. The specification’s Array.prototype.map algorithm shows how precisely that definition can describe one everyday method.
What is ECMAScript?
ECMAScript is the standardized language definition, and ECMA-262 is the specification that describes it. Ecma International’s living document states, “This Standard defines the ECMAScript 2027 general-purpose programming language.” It specifies syntax and semantics as well as built-in types, values, objects, properties and functions. The official living document consulted here identifies itself as the ECMAScript 2027 Language Specification, the eighteenth edition, with a draft heading dated October 1, 2026. It contains the latest yearly snapshot along with finished proposals that have reached Stage 4; because this document evolves, its edition and access date matter. Read the ECMAScript 2027 Language Specification.
Is JavaScript the same as ECMAScript?
The names are closely connected, but they refer to different things. JavaScript is the familiar language name and one of the originating technologies identified in the standard’s framing; ECMAScript is the standardized language definition. ECMA-262 describes the core language, not every capability available to a program. A host environment supplies its own objects and capabilities: for example, browser APIs belong to the browser environment rather than being part of core ECMAScript. This distinction helps explain why a language feature described in ECMA-262 can be used across environments while particular environment-specific facilities may differ.
What does the specification say about Array.prototype.map?
Section 23.1.3.21 of the ECMAScript 2027 indexed-collections chapter defines Array.prototype.map as an operation that calls a supplied callback for qualifying elements and builds a new Array from the callback results. The callback receives three arguments: the element value, its index, and the object being traversed. Calls occur in ascending index order, and the optional thisArg supplies the callback’s this value.
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“Qualifying elements” matters: the method calls the callback for indices that actually exist. A missing array element is skipped rather than passed to the callback, and the corresponding position in the result remains absent. The full algorithm is in ECMA-262’s indexed-collections chapter.
What happens if the array changes during mapping?
The range of indices to process is established before the first callback runs, but the method checks each index as it reaches it. Consequently:
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- An element appended beyond the initial range is not visited.
- If an existing element’s value changes before the method reaches its index, the callback sees the changed value.
- If an element is deleted before its turn, that index is skipped.
Does map change the original array?
map constructs and returns a new Array; the method itself does not directly mutate the array on which it was called. That is not a guarantee that mapping is side-effect-free: callback code can change the original array or other state. Use map when the callback’s results are intended to form a new array. By contrast, forEach is used for callback effects and does not construct a mapped-result array.
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