Explore how the `apply.call()` and `apply` methods differ in JavaScript. Get clear explanations of each method, an example, and practical applications to enhance your coding skills.
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Understanding the Key Differences Between apply.call() and apply in JavaScript
JavaScript can be a bit confusing at times, especially when it comes to understanding how different function calls work. Two of the lesser-known but powerful methods available in JavaScript are .apply() and .call(). Many developers find themselves puzzled when trying to figure out the difference between using .apply.call() and using .apply() directly. This guide aims to clarify these concepts with some concrete examples and explain the underlying mechanics in simple terms.
The Issue at Hand
When you're working with JavaScript functions, you might encounter scenarios where you need to control the value of this within a function. The .apply() and .call() methods are critical for achieving this. Let's take a look at an example that highlights the differences:
[[See Video to Reveal this Text or Code Snippet]]
At first glance, it may seem that these two statements might yield similar results, but they behave quite differently. Here's why.
Understanding .apply()
Normally, .apply() is invoked as follows:
[[See Video to Reveal this Text or Code Snippet]]
What this does is call someFunction, where thisValue becomes the value of this inside someFunction, and argsArray is an array of arguments passed to it.
Key Points About .apply():
thisValue: The context within which the function is executed.
argsArray: An array of parameters to pass to the function.
When we look at this line:
[[See Video to Reveal this Text or Code Snippet]]
It's essential to note that the apply() function is being called on the prototype of functions (Function.prototype), which always results in undefined. Therefore, even though it is supposed to call Math.floor, it does not actually work because the context (this) is wrong, leading us to receive undefined as a return value.
Understanding .call() and .apply.call()
Next, we analyze the first line of our original code which applies both .apply() and .call():
[[See Video to Reveal this Text or Code Snippet]]
How It Works:
Calling .call(): This begins our chain of function invocations. It attaches the ability of apply() to the Math.floor function.
Setting this: The this value when calling apply is Math.floor, which allows the function to be invoked correctly.
Passing Arguments: Lastly, it passes the arguments (in this case, [1.75]) correctly to Math.floor.
This is why the output in this case is 1, as Math.floor(1.75) correctly returns the floored value.
Conclusion
In summary, the differences between using .apply.call() and .apply() mainly revolve around how JavaScript determines the context of this. When using .apply.call(), you are chaining the functions and effectively controlling the context. On the other hand, simple usage of .apply() without the correct context will lead to unexpected results, as we saw with our example.
Final Remarks
Understanding these methods is essential for effective JavaScript programming, especially when dealing with functions in complex scenarios. So, the next time you find yourself using .apply() or .call(), you’ll be better equipped to get the results you expect.
Remember, mastering JavaScript’s many quirks is just part of the journey to becoming a more proficient developer.
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