为什么这段代码返回 true 而不是 false

Question

here is the code:

const str = "asdfcvb";
const result = str.split("").every((e) => {
    return e==='a'||'b'});
console.log(result);

I expect str.split("") returns an array, and every method check if every element in the array equals to 'a' or 'b', which should return false, but it returns true

Answer 1

Because e==='a'||'b' means ((e==='a')||'b')

When the e !== 'a' case, it will return 'b' which is truthy in Javascript, so every method got truthy value from each term and then the result will be true.

You may use the follow to achieve what you want

const result = str.split("").every((e) => {
    return (e==='a') || (e==='b')
});

Answer 2

You can't omit the === from the second part of your condition. Right now what you're saying is either return e === 'a' or 'b' . When e is not equal to 'a' it returns 'b' . Which is truthy and when turned into a boolean it becomes true. Run this in the browser:

'a' === 'b' || 'c'

If you run it you see that it returns 'c'. The same happens in your callback. You have to change your return statement to this for it to work:

return e === 'a' || e === 'b'

Answer 3

Everything seems correct. The problem is in the way you compared the strings

you should compare those like e === "a" || e === "b" e === "a" || e === "b"

and the working code

const str = "asdfcvb";
const result = str.split("").every((e) => {
    return (e === "a" || e === "b")});
console.log(result);

Explanation:

return e==='a'||'b'

running the test for the first item which is "a" will return true as now e is "a" here, which is equal to "a" .

now if it gets to second item which is "s" and obviously not equal to "a" so return e==='a'||'b' becomes return false||'b' and now when you evaluate this the returned value will be "b"

and putting "b" as a condition is similar to true cause it is not undefined or null

this can be tested as follows

if("b") return true; // returns true

Thus it returns true for every item thereby returning true at last

Answer 4

You are using the || operator in your case incorrectly. This is a common typo. You are supposed to use the same equal to phrase after ||. For example, your code would be like this:

 const str = 'asdfcvb'; const result = str.split('').every((e) => { return e === 'a' || e === 'b'; }); console.log(result);