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for (variable in object) {...
}

variable

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

for (var prop in obj) {
  console.log("obj." + prop + " = " + obj[prop]);
}

// :
// "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;

var obj = new ColoredTriangle();

for (var prop in obj) {
  if (obj.hasOwnProperty(prop)) {
    console.log("obj." + prop + " = " + obj[prop]);
  }
}

// :
// "obj.color = red"

:

SpiderMonkey 40, (i = 0) for...in:

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

40 , SyntaxError ("for-in loop head declarations may not have initializers") (Firefox bug 748550 Firefox bug 1164741).

, v8 (Chrome), Chakra (IE/Edge) JSC (WebKit/Safari) .

• for...of - ,
• for each in - , , ()
• for
• ( for...in)
• Object.getOwnPropertyNames()
• Object.prototype.hasOwnProperty()
• Array.prototype.forEach()