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Destructuring assignment

The two most used data structures in JavaScript are Object and Array.

  • Objects allow us to create a single entity that stores data items by key.
  • Arrays allow us to gather data items into an ordered list.

Although, when we pass those to a function, it may need not be an object/array as a whole. It may need individual pieces.

Destructuring assignment is a special syntax that allows us to “unpack” arrays or objects into a bunch of variables, as sometimes that’s more convenient.

Destructuring also works great with complex functions that have a lot of parameters, default values, and so on. Soon we’ll see that.

Array destructuring

Here’s an example of how an array is destructured into variables:

Now we can work with variables instead of array members.

It looks great when combined with split or other array-returning methods:

As you can see, the syntax is simple. There are several peculiar details though. Let’s see more examples, to better understand it.

“Destructuring” does not mean “destructive”.

It’s called “destructuring assignment,” because it “destructurizes” by copying items into variables. But the array itself is not modified.

It’s just a shorter way to write:

Ignore elements using commas

Unwanted elements of the array can also be thrown away via an extra comma:

In the code above, the second element of the array is skipped, the third one is assigned to title, and the rest of the array items is also skipped (as there are no variables for them).

Works with any iterable on the right-side

…Actually, we can use it with any iterable, not only arrays:

That works, because internally a destructuring assignment works by iterating over the right value. It’s a kind of syntax sugar for calling for..of over the value to the right of = and assigning the values.

Assign to anything at the left-side

We can use any “assignables” on the left side.

For instance, an object property:

Looping with .entries()

In the previous chapter we saw the Object.entries(obj) method.

We can use it with destructuring to loop over keys-and-values of an object:

The similar code for a Map is simpler, as it’s iterable:

Swap variables trick

There’s a well-known trick for swapping values of two variables using a destructuring assignment:

Here we create a temporary array of two variables and immediately destructure it in swapped order.

We can swap more than two variables this way.

The rest ‘…’

Usually, if the array is longer than the list at the left, the “extra” items are omitted.

For example, here only two items are taken, and the rest is just ignored:

If we’d like also to gather all that follows – we can add one more parameter that gets “the rest” using three dots "...":

The value of rest is the array of the remaining array elements.

We can use any other variable name in place of rest, just make sure it has three dots before it and goes last in the destructuring assignment.

Default values

If the array is shorter than the list of variables at the left, there’ll be no errors. Absent values are considered undefined:

If we want a “default” value to replace the missing one, we can provide it using =:

Default values can be more complex expressions or even function calls. They are evaluated only if the value is not provided.

For instance, here we use the prompt function for two defaults:

Please note: the prompt will run only for the missing value (surname).

Object destructuring

The destructuring assignment also works with objects.

The basic syntax is:

We should have an existing object on the right side, that we want to split into variables. The left side contains an object-like “pattern” for corresponding properties. In the simplest case, that’s a list of variable names in {...}.

For instance:

Properties options.titleoptions.width and options.height are assigned to the corresponding variables.

The order does not matter. This works too:

The pattern on the left side may be more complex and specify the mapping between properties and variables.

If we want to assign a property to a variable with another name, for instance, make options.width go into the variable named w, then we can set the variable name using a colon:

The colon shows “what : goes where”. In the example above the property width goes to w, property height goes to h, and title is assigned to the same name.

For potentially missing properties we can set default values using "=", like this:

Just like with arrays or function parameters, default values can be any expressions or even function calls. They will be evaluated if the value is not provided.

In the code below prompt asks for width, but not for title:

We also can combine both the colon and equality:

If we have a complex object with many properties, we can extract only what we need:

The rest pattern “…”

What if the object has more properties than we have variables? Can we take some and then assign the “rest” somewhere?

We can use the rest pattern, just like we did with arrays. It’s not supported by some older browsers (IE, use Babel to polyfill it), but works in modern ones.

It looks like this:

Gotcha if there’s no let

In the examples above variables were declared right in the assignment: let {…} = {…}. Of course, we could use existing variables too, without let. But there’s a catch.

This won’t work:

The problem is that JavaScript treats {...} in the main code flow (not inside another expression) as a code block. Such code blocks can be used to group statements, like this:

So here JavaScript assumes that we have a code block, that’s why there’s an error. We want destructuring instead.

To show JavaScript that it’s not a code block, we can wrap the expression in parentheses (...):

Nested destructuring

If an object or an array contain other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions.

In the code below options has another object in the property size and an array in the property items. The pattern on the left side of the assignment has the same structure to extract values from them:

All properties of options object except extra that is absent in the left part, are assigned to corresponding variables:

Finally, we have widthheightitem1item2 and title from the default value.

Note that there are no variables for size and items, as we take their content instead.

Smart function parameters

There are times when a function has many parameters, most of which are optional. That’s especially true for user interfaces. Imagine a function that creates a menu. It may have a width, a height, a title, items list and so on.

Here’s a bad way to write such function:

In real-life, the problem is how to remember the order of arguments. Usually IDEs try to help us, especially if the code is well-documented, but still… Another problem is how to call a function when most parameters are ok by default.

Like this?

That’s ugly. And becomes unreadable when we deal with more parameters.

Destructuring comes to the rescue!

We can pass parameters as an object, and the function immediately destructurizes them into variables:

We can also use more complex destructuring with nested objects and colon mappings:

The full syntax is the same as for a destructuring assignment:

Then, for an object of parameters, there will be a variable varName for property incomingProperty, with defaultValue by default.

Please note that such destructuring assumes that showMenu() does have an argument. If we want all values by default, then we should specify an empty object:

We can fix this by making {} the default value for the whole object of parameters:

In the code above, the whole arguments object is {} by default, so there’s always something to destructurize.

Summary

  • Destructuring assignment allows for instantly mapping an object or array onto many variables.
  • The full object syntax:let {prop : varName = default, ...rest} = objectThis means that property prop should go into the variable varName and, if no such property exists, then the default value should be used.Object properties that have no mapping are copied to the rest object.
  • The full array syntax:let [item1 = default, item2, ...rest] = arrayThe first item goes to item1; the second goes into item2, all the rest makes the array rest.
  • It’s possible to extract data from nested arrays/objects, for that the left side must have the same structure as the right one.

Tasks

Destructuring assignment

importance: 5

We have an object:

Write the destructuring assignment that reads:

  • name property into the variable name.
  • years property into the variable age.
  • isAdmin property into the variable isAdmin (false, if no such property)

Here’s an example of the values after your assignment:

solution

The maximal salary

importance: 5

There is a salaries object:

Create the function topSalary(salaries) that returns the name of the top-paid person.

  • If salaries is empty, it should return null.
  • If there are multiple top-paid persons, return any of them.

P.S. Use Object.entries and destructuring to iterate over key/value pairs.

Open a sandbox with tests.

solution

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