Just saw a line of code: a && b && c. I think this should be equivalent to if(a && b) {c}. But I'm really curious that is there any performance difference on these code, or it's just simply cool to put it in one line?

This is actual code: sourcesPanel && sourcesPanel.tips && sourcesPanel.tips.clearTimers()

The sourcesPanel and sourcesPanel.tips are two objects in the program. Basically, I think it's checking if those two objects exist, it they exist, call that clearTimers function.

Just saw a line of code: a && b && c. I think this should be equivalent to if(a && b) {c}. But I'm really curious that is there any performance difference on these code, or it's just simply cool to put it in one line?

This is actual code: sourcesPanel && sourcesPanel.tips && sourcesPanel.tips.clearTimers()

The sourcesPanel and sourcesPanel.tips are two objects in the program. Basically, I think it's checking if those two objects exist, it they exist, call that clearTimers function.

• javascript
• performance

• 1 Where did you see it? Depends on the context. – rafaelc Commented May 15, 2015 at 17:32
• if it was like if(a && b && c) then it means if all conditions are true then execute the next statement. – Abozanona Commented May 15, 2015 at 17:33
• @YazanWYusuf No it's not in if, just the line stand alone. – CodTango Commented May 15, 2015 at 17:35

3 Answers 3

The logical operators in JS have a very important behavior: they return the first conclusively failing or succeeding operand. If does not.

The behavior of if (a) { if (b) { ... is pretty simple: if they are truthy, enter the block.

The behavior of && is different:

foo = 1 && 2 && 3; // foo = 3, succeeds all the way through
foo = 1 && 0 && 3; // foo = 0, failed at 0
foo = 1 && null && 3; // foo = null, failed at null

When the logical operators are used within an if, the behavior appears to be exactly the same. Used on their own or with an assignment, they behave in a relatively unexpected way.

Take the use of || to provide defaults:

opt = opt || {foo: bar}; 

If opt was truthy (for an object, present) use it, otherwise use the defaults. The other logical operators behave the same way.

That is actually a double check and not a simple conditional check like you sugest: if(a && b) {c}.

The equivalent to a && b && c would be:

if (a && b){
    if(c){
        // do something

So all a, b and c have to be truthy.

a && b && c is mostly more practical than doing

if (a){
    if (b){
        if(c){
            // do something

But in a triple check like a && b && c b will never be evaluated if a evaluates false. Thus sparing time. The same for c that will only be evaluated if a and b also were true.

There is a performance improvement in some circumstances.

The line is executed step by step from left to right, and leaves as soon as it fails.

So, if a is false, b and c will not be evaluated.

You see this a lot in bash where one instruction is dependent upon another succeeding, but the first has to be attempted no matter what.