Analysis of JavaScript interview

some of the questions in Javascript make a lot of students feel headache, the following is based on brother Lian Education

Graduate students interview face questions, to make a simple share, I hope that the first into the workplace you have some help:JavaScript face question analysis.

First question

/*

parsing:

+ priority greater than ?

This problem is equivalent to: ' Value is true '? ' Something ': ' Nothing '

so the result:' Something '

*/

var val = ' Smtg ';

Console.log (' Value is ' + (val = = = ' Smtg ')? ' Something ': ' Nothing ');

Second question

/*

* Analysis:

* typeof Returns a String representing the type

See below for typeof results :

**type** **result**

Undefined "Undefined"

Null "Object"

Boolean "Boolean"

Number "number"

Symbol "symbol"

Hostobject implementation-dependent

function "function"

Object "Object"

The instanceof operator is used to detect if the Constructor.prototype exists on the prototype chain of the Parameter object

So output ["Object", false]

*/

Functiontwo () {

Console.info ([Typeofnull, null instanceof Object]); ["Object", false]

Third question

/*

[ sparse arrays and dense arrays in JavaScript ][1]

parsing:

in general, the arrays in JavaScript are sparse , that is , there can be gaps between the elements in the array

In fact , there is no regular array in JavaScript, and all arrays are actually an object.

the array of JavaScript is not indexed at all, because the index is a number, and the index of the array in JS is string,

ARR[1] In fact is arr["1", to arr["" "] =1,arr.length will automatically become 1001.

The root cause of these manifestations is that the object in JavaScript is a key-value pair of strings to any value . Note The key can only be a string .

Take a look at some of the Array.prototype.filter code :

var t =object (this);

var len =t.length >>> 0;

if (typeoffun!== ' function ') {

Thrownew TypeError ();

}

var res =[];

Varthisarg = Arguments.length >= 2? ARGUMENTS[1]: void 0;

for (var i= 0; i < len; i++) {

if (i int) {// note here !!!

Varval = T[i];

if (Fun.call (Thisarg, Val, I, T)) {

Res.push (Val);

}

}

}

from the above , when the filter array is traversed, it first checks whether the index value is an attribute of the array . Test it:

Console.info (0 in ary); True

Console.info (1 in ary); True

Console.info (4 in ary); False

Console.info (ten in ary); False

which means that 3~9 's index is not initialized at all.

so the answer:[];

*/

var ary = [0,1,2];

ARY[10] = 10;

Console.info (Ary.filter (function (x) {return x ===undefined;}));

Question Fourth

/*

parsing:

Y is assigned to the global . x is a local variable . So when you print X, it's referenceerror .

*/

(function () {

var x = y = 1;

})();

Console.log (y); 1

Console.log (x); Error

Question Fifth

/*

parsing:

when the function parameter involves any rest parameters,

Anydefault parameters or any destructured parameters ,

This arguments is not a mapped arguments object ...

so the answer is 12, and this needs to be well understood.

*/

function sideffecting (ary) {

Ary[0] =ary[2];

}

function Bar (a,b,c=3) {

c = 10

sideffecting (arguments);

Return a + B + C;

}

Bar (1,1,1);

Analysis of JavaScript interview