You can t even interrupt the JS array traversal and dare to call yourself a senior front end?

I interviewed a guy who claimed to be a senior front-end, proficient in JS, proficient in VUE, and worked for 6 years. We don't play with the hypothetical either, let him feelInterview to build rockets, work to screw up, then something real.

Q1:Do you know of any ways that JS can implement array traversal?

A1:for, for in, for of, foreach, map, etc., can all be used to traverse anyway".

Q2:So which of these methods can be interrupted?

A2:As if for can't, as if for in can ,..Incoherent, lost in thought.

Come on, ladies and gentlemen, if you don't know, then you can read this articleFavorites + LikesEncourage a wave.

forfor/infor/ofarray.foreacharray.maparray.reducearray.filterarray.somearray.Every method will not be nagged one by one, here is the key point.

Important: for in iterates over an object's enumerable properties other than symbol in any order. Therefore, for in is generally used to traverse an object. for of in iterable objects (inclusivearraymapsetstringtypedarray, arguments, objects, etc.), invoke custom iteration hooks, and execute statements for the values of each different property. Therefore, for of is generally used to iterate over arrays. Note:

for in is unordered, you need to pay attention to the use of traversal, refer to: js puzzle-object in the key is ordered?What isEnumerable？Take a closer look: Enumerable Highlight Examples:

Both to traverse the object let obj = ;// for/infor (var o in obj) ;// output// id// name// for/offor (var o of obj) ;The Error object is not an iterator and cannot be used for of VM319:1 Uncaught TypeError: Obj is not iterable

Both iterate over the array let ary = ['a', 'b', 'c'];Note: Arrays are also a special object, and I add a property aryname = 'james'; // for/infor (var o in ary) ;output prints the array index and the value of the name property (note)!// 0// 1// 2// james// for/offor (var o of ary) ;output prints only the array for each element, a, b, c

Important: create a new array (don't forget to putmapThe result is assigned to the traversal, otherwise a traversal is wasted) The original array is not affected (unless you put themapWhenforeachuse, modify the value of the original array directly in the loop) each result of the new array is the value returned in the ** function (if your ** function forgets).retureEach element of the new array isundefinedTypical Error Cases:

putmapUsed to iterate through arrays,map, to directly modify the value of the original array

Example of error:

let demoary = [,demoary.map(item => )console.log(demoary);

let demoary = [, method 1: foreach iterate through the array demoaryforeach(item => )console.log(demoary);Method 2: map generates a new array to replace the original array demoary = demoarymap(item => )console.log(demoary);

Produces a new result, and the result depends on the ** functionreturnThe value of the original array is not affected (unless you put itreduceWhenforeachuse, modify the value of the original array directly in the loop) to receive 2 arguments,callbackwithinitialvaluecallbackThere are 4 parameters:Cumulative valueCurrent valueCurrent indexRaw value, generally using the first 2 parametersinitialvalueOptionally, if you omit the default 1st element, but if the array is also empty, you will get an error with the important example:

There are initial values [1, 2, 3, 4, 5].reduce((acc, cur) => ,100);output iterates 5 times and finally returns the int sum of 115 100 1 101 2 103 3 106 4 110 5 115

There are no initial values [1, 2, 3, 4, 5].reduce((acc, cur) => ) output iterates 4 times, and finally returns the int sum result of 15 1 2 3 3 6 4 10 5 15

Highlights Example:

// some[1, 2, 3, 4, 5].some(item => ;The default is false and can be omitted. It means that it cannot be found, and it will continue to traverse and find the last return false; of the array}) output iterate 3 times, find the result and return true 1 2 3 true

// every[1, 2, 3, 4, 5].every(item => ;The default is false, which must be written. If you don't explicitly state that this time is true, it means that the first element is not suitable, and it will be exited directly.  return true;}) output iterate 3 times, find an inappropriate result and return false 1 2 3 false

BreakThrowReturnContinue (immediately end this loop, continue after ** does not execute, enter the next round of loop, it is considered to abort the current loop, and it is barely counted) forfor ofarraysomearray.Every of these methods we want:InterruptionsA common method for an array isarray.somewitharray.every, because you have to meet certain conditions before you break an array traversal, right?It might be more appropriate to use these 2 syntactic sugars.

// for breaklet ary = [1, 2, 3, 4, 5];for (let i=0; i< ary.length; i++)// output// 0// 1// 2// 3// --// for continuefor (let i=0; i< ary.length; i++)console.log('after=', i);} output Note that there is no after=3 before= 0 after= 0 before= 1 after= 1 before= 2 after= 2 before= 3 before= 4 after= 4

// for/of breaklet ary = [1, 2, 3, 4, 5];for (let o of ary) ;// output// 1// 2// 3// --// for/of continuefor (let o of ary) ;console.log('after=', o);} output Note that there is no after=3 before= 1 after= 1 before= 2 after= 2 after= 2 before= 3 before= 4 after= 4 before= 5 after= 5

These 2 methods are related to:forwithfor/ofThe difference to note is that they break the syntaxreturnIt cannot be usedbreakorcontinue