with

with (expression)
  statement

expression

exrpession statement. .

statement

. , block statement ({ ... }), .

JavaScript unqualified , , , unqualified . 'with' . unqualified , , . ReferenceError.

Performance pro & contra

Pro: with , . , 'with', . 'with' . , , .

Contra: with . , , with. , , with , .

Ambiguity contra

Contra: 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. So given this example:

function f(x, o) {
  with (o) {
    console.log(x);
  }
}

Only when f is called is x either found or not, and if found, either in o or (if no such property exists) in f's activation object, where x names the first formal argument. If you forget to define x in the object you pass as the second argument, or if there's some similar bug or confusion, you won't get an error just unexpected results.

Contra: Code using with may not be forward compatible, especially when used with something other than a plain object. Consider this example:

function f(foo, values) {
  with (foo) {
    console.log(values);
  }
}

If you call f([1,2,3], obj) in an ECMAScript 5 environment, then the values reference inside the with statement will resolve to obj. However, ECMAScript 6 introduces a values property on Array.prototype (so that it will be available on every array). So, in a JavaScript environment that supports ECMAScript 6, the values reference inside the with statement will resolve to [1,2,3].values.

with

with Math . with PI [cos]](/ru/docs/Web/JavaScript/Reference/Global_Objects/Math/cos) [sin]](/ru/docs/Web/JavaScript/Reference/Global_Objects/Math/sin), . JavaScript Math .

var a, x, y;
var r = 10;

with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}

• block
• Strict mode
• Symbol.unscopables
• Array.prototype[@@unscopables]