将javascript中的逻辑操作数与对象一起使用

Question

I came across the use of the logical operand && with objects however would like to know whether expressions are evaluated in the manner below only when dealing with objects.

Code Sample

var sampleObject = {
            size: 10, 
            shoe: {
                make: "nike",
                model: "air jordon"
            },
            color: "red"
            }

    console.log(sampleObject.size && sampleObject.shoe.model);

The out of console.log(sampleObject.size && sampleObject.shoe.model); returns air jordon . My understanding is that because the first condition ie sampleObject.size evaluated to true and isn't falsely it returns the value contained in the object sampleObbject.shoe.model .

Answer 1

The Javascript && operator first evaluates its left operand. If it's a falsey value, it is the value of the entire expression. If it's a truthy value, then the right operand is evaluated, and it is returned.

Zeros, empty strings, undefined, and false are all falsey values. Your sampleObject.size value is 10, a truthy value, so the right operand is evaluated and used as the value of the entire expression.

Note that none of this depends on the values being objects.

Answer 2

The || and && operators run from left to right. In the case of || , the first truthy value is returned. For && , it's the first falsy value. In both cases, the last one is returned if no suitable return value is found.

So, in this case, the fact that it returned a truthy value for the expression means that both sampleObject.size and sampleObject.show.model are truthy values.