with (expression)
  statement

expression

Adds the given expression to the scope chain used when evaluating the statement. The parentheses around the expression are required.

statement

Any statement. To execute multiple statements, use a block statement ({ ... }) to group those statements.

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.

foo; // unqualified identifier
foo.bar; // bar is a qualified identifier

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.

const foo = { bar: 1 };
console.log(foo.bar);
// foo is found in the scope chain as a variable;
// bar is found in foo as a property

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.

console.log(globalThis.Math === Math); // true

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 a 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.

with ([1, 2, 3]) {
  console.log(toString()); // 1,2,3
}

The object may have an @@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:
  js

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

  If you look just at the definition of f, it's impossible to tell what the x in the with body refers to. Only when f is called can x be determined to be o.x or f's first formal parameter. If you forget to define x in the object you pass as the second parameter, you won't get an error — instead you'll just get unexpected results. It's also unclear what the actual intent of such code would be.
• Forward compatibility: Code using with may not be forward compatible, especially when used with something other than a plain object, which may gain more properties in the future. Consider this example:
  js

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

  If you call f([1, 2, 3], obj) in an ECMAScript 5 environment, the values reference inside the with statement will resolve to obj. However, ECMAScript 2015 introduces a values property on Array.prototype (so it will be available on every array). So, after upgrading the environment, the values reference inside the with statement resolves to [1, 2, 3].values instead, and is likely to cause bugs. In this particular example, values is defined as unscopable through Array.prototype[@@unscopables], so it still correctly resolves to the values parameter. If it were not defined as unscopable, one can see how this would be a difficult issue to debug.

The following with statement specifies that the Math object is the default object. The statements following the with statement refer to the PI property and the cos and sin methods, without specifying an object. JavaScript assumes the Math object for these references.

let a, x, y;
const r = 10;

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

You can usually avoid using with through property destructuring. Here we create an extra block to mimic the behavior of with creating an extra scope — but in actual usage, this block can usually be omitted.

let a, x, y;
const r = 10;

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

If you're producing an expression that must reuse a long-named reference multiple times, and your goal is to eliminate that lengthy name within your expression, you can wrap the expression in an IIFE and provide the long name as an argument.

const objectHavingAnEspeciallyLengthyName = { foo: true, bar: false };

if (((o) => o.foo && !o.bar)(objectHavingAnEspeciallyLengthyName)) {
  // This branch runs.
}

with will transform every variable lookup to a property lookup, while Proxies allow trapping every property lookup call. You can create a dynamic namespace by combining them.

const namespace = new Proxy(
  {},
  {
    has(target, key) {
      // Avoid trapping global properties like `console`
      if (key in globalThis) {
        return false;
      }
      // Trap all property lookups
      return true;
    },
    get(target, key) {
      return key;
    },
  },
);

with (namespace) {
  console.log(a, b, c); // "a" "b" "c"
}

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