为什么此.every（）函数返回true然后返回false？

Question

let nums = [-1, 50, 75, 200, 350, 525, 1000];

nums.every(function(num) {
  console.log(num < 0);
});

true

=> false

---

When I run this code in https://repl.it/@super8989/BraveFunctionalSale , this returns "true" then "=> false".

According to .every() description, the return value is "true if the callback function returns a truthy value for every array element; otherwise, false."

Why does it show "true" and then "=> false"?

---

Furthermore, when I change the array so that "- value" is in the middle of the array, it returns "false" then "=> false".

let nums = [1, 50, -75, 200, 350, 525, 1000];

nums.every(function(num) {
  console.log(num < 0);
});

false

=> false

https://repl.it/@super8989/CyberInterestingPhase

---

let nums = [-1, 50, 75, 200, 350, 525, 1000];

console.log(nums.every(num => num < 0));

false

=> undefined

But if I write it this way, this returns false then undefined. https://repl.it/@super8989/MonstrousAjarDimension

---

I'm very confused... Please help!

Answer 1

You need to use return inside .every, currently you are just logging num < 0 which is true for num[0] which is (-1 < 0), but false for num[1] which is (50 >= 0). And since you do not return is instantly exits the loop as this is considered false .

 let numsA = [-1, 50, 75, 200, 350, 525, 1000]; console.log("should return false, because 50 is >= 0"); console.log(numsA.every(function(num) { return (num < 0); })); let numsB = [1, 50, -75, 200, 350, 525, 1000]; console.log(numsB.every(function(num) { return (num < 0); })); let numsC = [-1, 50, 75, 200, 350, 525, 1000]; console.log(numsC.every(num => num < 0)); let numsD = [-5, 0, 5, 10, 15]; console.log("should return true because for each num in numsD, num %5 === 0"); console.log(numsD.every(num => { return num % 5 === 0 })); //you can omit return with arrow function syntax, as this automatically returns the value within () console.log(numsD.every(num => (num % 5 === 0))); 

Answer 2

In Array.every method you must provide function that returns true or false . Function is being called for every one element in Array. https://developer.mozilla.org/pl/docs/Web/JavaScript/Referencje/Obiekty/Array/every

your function

nums.every(function(num) {
  console.log(num < 0);
});

returns undefined for every element (missing return statement).

var resutl = nums.every(function(num) {
  return (num < 0);
});

or in es-2017 syntax:

const resutl = nums.every(num => num < 0);