74 lines
1.6 KiB
Markdown
74 lines
1.6 KiB
Markdown
---
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title: For...In Loop
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---
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The `for...in` statement iterates over the enumerable properties of an object, in an arbitrary order. For each distinct property, statements can be executed.
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```js
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for (variable in object) {
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...
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}
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```
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| Required/Optional | Parameter | Description |
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|-------------------|-----------|----------------------------------------------------------------------|
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| Required | Variable | A different property name is assigned to variable on each iteration. |
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| Optional | Object | Object whose enumerable properties are iterated. |
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## Examples
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```js
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// Initialize object.
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const a = { "a": "Athens", "b": "Belgrade", "c": "Cairo" };
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// Iterate over the properties.
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let s = "";
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for (let key in a) {
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s += key + ": " + a[key];
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s += "<br />";
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}
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document.write (s);
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// Output:
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// a: Athens
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// b: Belgrade
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// c: Cairo
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// Initialize the array.
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let arr = new Array("zero", "one", "two");
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// Add a few expando properties to the array.
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arr["orange"] = "fruit";
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arr["carrot"] = "vegetable";
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// Iterate over the properties and elements.
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let s = "";
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for (let key in arr) {
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s += key + ": " + arr[key];
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s += "<br />";
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}
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document.write (s);
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// Output:
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// 0: zero
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// 1: one
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// 2: two
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// orange: fruit
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// carrot: vegetable
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// Efficient way of getting an object's keys using an expression within the for-in loop's conditions
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var myObj = {a: 1, b: 2, c:3}, myKeys = [], i=0;
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for (myKeys[i++] in myObj);
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document.write(myKeys);
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//Output:
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// a
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// b
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// c
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```
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## Other Resources:
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* [MDN link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in)
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* [MSDN link](https://msdn.microsoft.com/library/55wb2d34.aspx)
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