How to create an array if an array does not exist yet?

How do I create an array if it does not exist yet? In other words how to default a variable to an empty array?

javascript

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You can use the typeof operator to test for undefined and the instanceof operator to test if it’s an instance of Array:

if (typeof arr == "undefined" || !(arr instanceof Array)) {
var arr = [];
}

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If you are talking about a browser environment then all global variables are members of the window object. So to check:

if (window.somearray !== undefined) {
somearray = [];
}

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var arr = arr || [];

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最佳答案

If you want to check whether an array x exists and create it if it doesn't, you can do

x = ( typeof x != 'undefined' && x instanceof Array ) ? x : []

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If you want to check if the object is already an Array, to avoid the well known issues of the instanceof operator when working in multi-framed DOM environments, you could use the Object.prototype.toString method:

arr = Object.prototype.toString.call(arr) == "[object Array]" ? arr : [];

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const list = Array.isArray(x) ? x : [x];

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

Or if x could be an array and you want to make sure it is one:

const list = [].concat(x);

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

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<script type="text/javascript">


array1  = new Array('apple','mango','banana');
var storearray1 =array1;


if (window[storearray1] && window[storearray1] instanceof Array) {
alert("exist!");
} else {
alert('not find: storearray1 = ' + typeof storearray1)
}


</script>