See also on the official Mozilla Website - Assigning a default value to a variable. However, this has again a diminished cross-browser support. This is the nullish assignment which applies only to values that are null or undefined ( nullish) - of which the empty string is not part. This will be better supported across browsers also.Īlternatively you could write this as: obj.d ?= "default" If it is important not to override those, you might need to rewrite this as: if (typeof obj.d = "undefined") obj.d = "default" So two troublesome cases are the empty String "" and 0 (zero). Note that this is not uniformly supported across browsers and nodejs versions (confirm for yourself). Most of the time, this happens between the stages 3 and 4, and is usually before the spec is officially published.This works because undefined || "1111111" -> "1111111"Īs null, undefined, NaN, 0, "" (Empty String), false itself, are all considered to be equivalent to false ( falsy). This means that cases where some proposals for new ECMAScript features have already been implemented in browsers, documentation and examples in MDN articles may use some of those new features. As soon as one browser implements a feature, we try to document it. The standards for JavaScript are the ECMAScript Language Specification (ECMA-262) and the ECMAScript Internationalization API specification (ECMA-402). For information about APIs that are specific to Web pages, please see Web APIs and DOM. This section is dedicated to the JavaScript language itself, and not the parts that are specific to Web pages or other host environments. JavaScript's dynamic capabilities include runtime object construction, variable parameter lists, function variables, dynamic script creation (via eval), object introspection (via for.in and Object utilities), and source-code recovery (JavaScript functions store their source text and can be retrieved through toString()). JavaScript is a prototype-based, multi-paradigm, single-threaded, dynamic language, supporting object-oriented, imperative, and declarative (e.g. While it is most well-known as the scripting language for Web pages, many non-browser environments also use it, such as Node.js, Apache CouchDB and Adobe Acrobat. JavaScript ( JS) is a lightweight interpreted (or just-in-time compiled) programming language with first-class functions. Warning: unreachable code after return statement.Warning: -file- is being assigned a //# sourceMappingURL, but already has one.TypeError: X.prototype.y called on incompatible type.TypeError: setting getter-only property "x".TypeError: Reduce of empty array with no initial value.
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