有人可以在javascript中解释这种行为吗？

Question

Tested in Firefox:

a = [] + 0;
b = 0;

alert(a); // 0
alert(b); // 0

alert(!a); // false
alert(!b); // true

Answer 1

The + concatenation operator causes a toString evaluation of 0 .

As such the value of a is "0" , while the value of b is 0 .

From ECMAScript 11.6.1 The Addition operator(+)

The addition operator either performs string concatenation or numeric addition. The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows:

1. Let lref be the result of evaluating AdditiveExpression.

2. Let lval be GetValue(lref).

3. Let rref be the result of evaluating MultiplicativeExpression.

4. Let rval be GetValue(rref).

5. Let lprim be ToPrimitive(lval).

6. Let rprim be ToPrimitive(rval).

7. If Type(lprim) is String or Type(rprim) is String, then

   a. Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

8. Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See the Note below 11.6.3.

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.

NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the logical-or operation instead of the logical-and operation.

Answer 2

[] + 0 is a string.
When you write a + b in Javascript, the engine will convert both a and b to primitives.
If at least one of the resulting primitives is a string, it will perform string concatenation; otherwise, it will perform numeric addition.

To convert an object (such as an array) to a primitive, the engine will call valueOf() , and, if its result is not a primitive, will call toString() .
For arrays, valueOf() returns the original array (which is not a primitive) and toString() returns comma-separated string of the array's contents.

Therefore, [] + 0 becomes "" + 0 , which is "0" .

Non-empty strings are never falsy.