为什么你可以检查Javascript中是否存在.length的数组

Question

As I Java programmer I do not understand why you can check if the array is empty using if(array.length) . Can someone please elaborate?

Answer 1

because first if array is not defined (undefined) then you can't get the length property of an undefined object.

just use if (typeof array !== 'undefined') will give the result if array exsit.

as you can see in the example code:

zero.length --> 0
!zero.length --> true
!!zero.length --> false

0 is considered as false in if(0) or !!0 / !0 , since an empty array have length of 0, it is considered as false, in that case you can't use it to determine if this array exsit or it is just empty .

 var array = []; if (typeof array !== 'undefined'){ console.log('array exsit: true'); } else { console.log('array exsit: false'); } var zero = []; console.log('zero.length --> '+zero.length); console.log('!zero.length --> '+!zero.length); console.log('!!zero.length --> '+!!zero.length); if (typeof array2 !== 'undefined'){ console.log('array2 exsit: true'); } else { console.log('array2 exsit: false'); } 

below is the table for js truthy and falsy

 function checkTruthiness(raw, exp) { row = $('<tr />').appendTo('#output'); if ( exp ) { row.append('<td class="truthy">' + raw + '</td>'); } else { row.append('<td class="falsy">' + raw + '</td>'); } if ( !exp ) { row.append('<td class="truthy"> !' + raw + '</td>'); } else { row.append('<td class="falsy"> !' + raw + '</td>'); } if ( exp == true ) { row.append('<td class="truthy">' + raw + ' == true </td>'); } else { row.append('<td class="falsy">' + raw + ' == true </td>'); } if ( exp == false ) { row.append('<td class="truthy">' + raw + ' == false </td>'); } else { row.append('<td class="falsy">' + raw + ' == false </td>'); } if ( exp === true ) { row.append('<td class="truthy">' + raw + ' === true </td>'); } else { row.append('<td class="falsy">' + raw + ' === true </td>'); } if ( exp === false ) { row.append('<td class="truthy">' + raw + ' === false </td>'); } else { row.append('<td class="falsy">' + raw + ' === false </td>'); } if ( Boolean(exp) ) { row.append('<td class="truthy">Boolean(' + raw + ')</td>'); } else { row.append('<td class="falsy">Boolean(' + raw + ')</td>'); } } checkTruthiness('0', 0); checkTruthiness('NaN', NaN); checkTruthiness('""', ""); checkTruthiness('false', false); checkTruthiness('null', null); checkTruthiness('undefined', undefined); checkTruthiness('{}', {}); checkTruthiness('[]', []); checkTruthiness('"0"', "0"); checkTruthiness('"false"', "false"); 

 td { font-family: Verdana, sans-serif; font-size:13px; padding:5px 10px; } .truthy { background-color:#cfc; } .falsy { background-color:#fcc; } 

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <table id="output"></table> 

Answer 2

Actually array.length is evaluated as true when it has at least one element. When array is empty array.length returns 0 which is evaluated as false .

So the bottom line is you can use if (array.length) to check if array is NOT empty.

Answer 3

array.length will return 0 if the array is empty, 0 is a falsy value.

Falsy values

Examples of falsy values in JavaScript (which translate to false and thus bypass the if block):

• false
• 0 (zero)
• "" (empty string)
• null
• undefined
• NaN

So

var arr = [];

arr.length === 0
arr.length == false
!arr.length === true

Any non-zero number is a truthy value, and will enter the if block.

Answer 4

零强制为false ：

 if (!0) console.log(true); 

Answer 5

You can dynamically calculate the number of elements in the array through the lenght property (the same property can also be applied to a string to count its characters, in the case of the Array () the number of indexes is calculated). Since .lenght returns the number of array elements, if it is empty you will have 0 as the return value.

Answer 6

In most of the programming languages, 0 and null ( undefined in case of JavaScript) are equivalent to false . And thus that if condition is correct.

Unless you want totally different behavior for 0 and null , then you'll have to do check using == ( === in case of JS). This statement returns boolean value.

JavaScript considers NaN & Empty string also as false.