JS：为什么这返回true？

Question

0 + 1 === 1 || 1 + 1 === 2 || 2 + 1 === 10

Can someone please explain why this statement returns true ?

Individually, the first two are true and the last one is false. However OVERALL , the statement returns true. It seems that you just need one true statement among a longer list of statements to make the OVERALL statement return true. Is that correct?

Just trying to cement my understanding of Booleans. Cheers.

Answer 1

Well, || returns the first truthy value if there is one so:

1 || ANYTHING_IN_THE_WORLD ; // returns 1

So all you really have there is

0 + 1 === 1

Which is true. So yes, your understanding is correct.

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Expanding on execution order:

Operator precedence is in play here:

0 + 1 === 1 || 1 + 1 === 2 || 2 + 1 === 10

First we have addition:

1 === 1 || 2 === 2 || 3 === 10

Then we have equality checks:

true || true || false;

Which as explained above true || ANYTHING_AT_ALL true || ANYTHING_AT_ALL is true, so the final output is true .

Answer 2

|| is an or operator. So if condition one or condition two or condition 3 is true, the statement is true.

I suggest reading up on JavaScript here: https://developer.mozilla.org/en/docs/Web/JavaScript

Answer 3

it returns true because with OR operator (||) first true will return true.

The evaluation will perform :

> 1- is (0+1===1)

=> true! Then returns true.

This

> 0+1===1 || 1+1000===1

will return true for the same reason.

Answer 4

"some apples are purple OR some apples are green" is true because it is sufficient that some apples are green. a OR b will be true when either or both are true. a AND b needs both to be true.

Thus, the fact that some of the items in your expression are true makes the whole expression true. It would not be if you replaced || with && .

In fact,

It seems that you just need one true statement among a longer list of statements to make the OVERALL statement return true. Is that correct?

is (almost) exactly correct. The only correction I'd put in is replacing "true" with "truthy", at least in the context of JavaScript. For example, 3 || 5 3 || 5 is not true , but 3 - while 3 is not true , it is truthy.