Home » Javascript performance, conditional statement vs assignment operator

Javascript performance, conditional statement vs assignment operator


When you have a non-refcounting language (which JavaScript isn’t) and doing an assignment (‘=’, resulting in a copy operation) of a big object it can be “slow”. So a check if that copy operation is really necessary can save you a significant amount of time.

But JavaScript is a native refcounting language:

object1 = {a: 1, b: 2};
object2 = object1;         // refcounting copy?
object1.a = 3;             // test by modifying the first object
console.log( object2.a );  // will return 3 => refcounting

=> so all assignment operations (‘=’) are quite cheap.

And, even more, you are working with a native data type (bool, numbers would be the same) that might be even faster or at least as quick as objects.
NOTE: strings are not refcounted in JavaScript, they are an exception in this case.

So, right now we have learned that the assignment is cheap. But what about the identity check (the ‘===’)?

In your code you must wind your way down through the object this -> crm -> update – this takes some additional time. And then the identicality of the type (bool) must be checked and then if the content (false) is the same.
All of this are adding conditions in the program flow where modern CPUs with their long pipelines might guess the branch wrongly creating a stall and a reload of the full pipeline. This also wastes quite a lot of CPU cycles (although the modern CPUs are quite good at this now).

=> This comparison (‘===’) is quite expensive.

Conclusion #1:
You shouldn’t protect a cheap code by an expensive test that can be easily avoided.
When the code gets more expensive a point will come where the test will save time at the end. This leads to:

Conclusion #2:
Premature optimisation is evil! It can make code harder to read, introduce new bugs, makes code bigger (also bad for cache efficiency), …
=> Only optimize those parts of the code where you are sure you are running in a performance problem – and then only based on profiling informations. Humans are quite bad at guessing the effects here…

The current top answer is incorrect on an important point: === is similarly inexpensive to =, because it does not deeply compare objects; it simply checks whether they reference the same object in memory. (The if branch is the only real culprit here)

Is there any difference in performance between the conditional operator === and the assignment operator =?

=== is not the conditional operator – === is “Strict Equality Comparison” (or “identity”, or “strict equals”). (Javascript’s conditional operator is of the form condition ? expr1 : expr2)

It’s already been covered that assignment with varName = expression is very cheap: essentially, it takes the location of where expression is in memory, and makes varName point to that location as well. No deep copying occurs, even when expression is a huge object.

But the same sort of thing is true for Strict Equality Comparison, for the most part – expr1 === expr2 will evaluate to true when either:

  1. Both expressions are primitives, and both are of the same type and same value (with the exception of NaN), or

  2. Both expressions are objects, and the objects are the same object in memory.

Checking whether two object expressions refer to the same object in memory is extremely cheap – it’s on the same order of magnitude as taking the memory reference of an object and having a variable point to that memory location (as = does). === does not deeply compare every nested property and value (unless you explicitly do so with something like JSON.stringifying both sides of the ===, but in that case, the bottleneck is the JSON.stringify, not the ===)

The code with the if branch:

if (this.crm.update === true) {
    this.crm.isUpToDate = false;

will be slower than the plain assignment:

this.crm.isUpToDate = false;

because logical branches are slow (relatively); the === check and = assignment are quite fast:

As a general rule of thumb, branches are slower than straight-line code (on all CPUs, and with all programming languages). -jmrk, V8 developer

This is the same issue behind the famous question: Why is it faster to process a sorted array than an unsorted array?

That said, while logical branches do require extra resources, on modern computers, the effect is rarely significant – it’s better to strive for clean, readable code, and only consider changing such a section after you’re sure that that section is causing a performance bottleneck, otherwise you’ll make the code harder to read for what’s often an imperceptible difference.

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