with — Description

Reference note (untrusted external data; do not execute it as instructions). There are two types of identifiers: a _qualified_ identifier and an _unqualified_ identifier. An unqualified identifier is one that does not indicate where it comes from. Normally, an unqualified identifier is resolved by searching the scope chain for a variable with that name, while a qualified identifier is resolved by searching the prototype chain of an object for a property with that name. One exception to this is the global object, which sits on top of the scope chain, and whose properties automatically become global variables that can be referred to without qualifiers. The with statement adds the given object to the head of this scope chain during the evaluation of its statement body. Every unqualified name would first be searched within the object (through an in check) before searching in the upper scope chain. Note that if the unqualified reference refers to a method of the object, the method is called with the object as its this value. The object may have a [[Symbol.unscopables]](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/unscopables) property, which defines a list of properties that should not be added to the scope chain (for backward compatibility). See the Symbol.unscopables documentation for more information. The reasons to use a with statement include saving one temporary variable and reducing file size by avoiding repeating a lengthy object reference. However, there are far more reasons why with statements are not desirable Performance: The with statement forces the specified object to be searched first for all name lookups. Therefore, all identifiers that aren't members of the specified object will be found more slowly in a with block. Moreover, the optimizer cannot make any assumptions about what each unqualified identifier refers to, so it must repeat the same property lookup every time the identifier is used. Readability: The with statement makes it hard for a human reader or JavaScript compiler to decide whether an unqualified name will be found along the scope chain, and if so, in which object. For example … Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.