for...in

const object = { a: 1, b: 2, c: 3 };

for (const property in object) {
  console.log(`${property}: ${object[property]}`);
}

// Expected output:
// "a: 1"
// "b: 2"
// "c: 3"

for (const variable in object) {
  statement;
}

variable

(Value name) (variable) .

object

.

for...in non-Symbol .

Array Object constructor String indexOf(), Object toString() Object.prototype, String.prototype . for...in . ( .)

, ,

for..in . ( cross-browser delete .)

, . . .

, , . , ( ) , .

for...in

Note: for...in Array .

, . for...in . for...in , .

, . for .( Array.prototype.forEach(), for...of .)

getOwnPropertyNames() hasOwnProperty() .(propertyIsEnumerable() .) .

for...in ?

for...in , , Array.prototype.forEach(), for...of . for...in ?

( ) . , - ( "key" ) for...in .

for...in

non-Symbol .

var obj = { a: 1, b: 2, c: 3 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
}

// Output:
// "obj.a = 1"
// "obj.b = 2"
// "obj.c = 3"

hasOwnProperty() . .

var triangle = { a: 1, b: 2, c: 3 };

function ColoredTriangle() {
  this.color = "red";
}

ColoredTriangle.prototype = triangle;

function show_own_props(obj, objName) {
  var result = "";

  for (var prop in obj) {
    if (obj.hasOwnProperty(prop)) {
      result += objName + "." + prop + " = " + obj[prop] + "\n";
    }
  }

  return result;
}

o = new ColoredTriangle();
alert(show_own_props(o, "o")); /* alerts: o.color = red */

Compatibility: Initializer expressions in strict mode

Prior to Firefox 40, it was possible to use an initializer expression (i=0) in a for...in loop:

const obj = { a: 1, b: 2, c: 3 };
for (var i = 0 in obj) {
  console.log(obj[i]);
}
// 1
// 2
// 3

This nonstandard behavior is now ignored in version 40 and later, and will present a SyntaxError ("for-in loop head declarations may not have initializers") error in strict mode (bug 748550 and bug 1164741).

Other engines such as v8 (Chrome), Chakra (IE/Edge), and JSC (WebKit/Safari) are investigating whether to remove the nonstandard behavior as well.

See also

• for...of a similar statement that iterates over the property values
• for
• Iterators and Generator functions (usable with for...of syntax)
• Enumerability and ownership of properties
• Object.getOwnPropertyNames()
• Object.hasOwn()
• Array.prototype.forEach()