121 lines
3.7 KiB
Markdown
121 lines
3.7 KiB
Markdown
# Variable Declarations
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Declaring a variable in JavaScript has always traditionally been done with the `var` keyword.
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```typescript
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var a = 10;
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```
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The `var` construct has some [problems](http://www.typescriptlang.org/docs/handbook/variable-declarations.html),
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which is why `let` statements were introduced. Apart from the keyword used, `let` statements are written
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the same way `var` statements are.
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```typescript
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let a = 10;
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```
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The key difference is not in the syntax, but in the semantics, which we'll now dive into.
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## Block-scoping
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When a variable is declared using `let`, it uses what some call *lexical-scoping* or *block-scoping*.
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Unlike variables declared with `var` whose scopes leak out to their containing function,
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block-scoped variables are not visible outside of their nearest containing block or `for`-loop.
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```typescript
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function f(input: boolean) {
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let a = 100;
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if (input) {
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// Still okay to reference 'a'
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let b = a + 1;
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return b;
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}
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// Error: 'b' doesn't exist here
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return b;
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}
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```
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Here, we have two local variables `a` and `b`.
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`a`'s scope is limited to the body of `f` while `b`'s scope is limited to the containing `if` statement's block.
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Another property of block-scoped variables is that they can't be read or written to before they're actually declared.
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While these variables are "present" throughout their scope, all points up until their declaration are part of their *temporal dead zone*.
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This is just a sophisticated way of saying you can't access them before the `let` statement, and luckily TypeScript will let you know that.
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```typescript-ignore
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a++; // illegal to use 'a' before it's declared;
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let a;
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```
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## Re-declarations
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With `var` declarations, it doesn't matter how many times you declare your variables, you just get one:
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```typescript
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var x = 10;
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var x = 20;
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```
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In the above example, all declarations of `x` actually refer to the *same* `x`, and this is perfectly valid.
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This often ends up being a source of bugs. Thankfully, `let` declarations are not as forgiving.
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```typescript
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let x = 10;
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let x = 20; // error: can't re-declare 'x' in the same scope
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```
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## Shadowing
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The act of introducing a new name in a more deeply nested scope is called *shadowing*.
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It is a bit of a double-edged sword in that it can introduce certain bugs on its own in the
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event of accidental shadowing, while also preventing certain bugs.
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For instance, imagine a `sumMatrix` function using `let` variables.
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```typescript
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function sumMatrix(matrix: number[][]) {
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let sum = 0;
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for (let i = 0; i < matrix.length; i++) {
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var currentRow = matrix[i];
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for (let i = 0; i < currentRow.length; i++) {
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sum += currentRow[i];
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}
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}
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return sum;
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}
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```
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This version of the loop will actually perform the summation correctly because the inner loop's `i` shadows `i` from the outer loop.
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Shadowing should *usually* be avoided in the interest of write clearer code, such as
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```typescript
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function sumMatrix(matrix: number[][]) {
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let sum = 0;
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for (let i = 0; i < matrix.length; i++) {
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var currentRow = matrix[i];
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for (let j = 0; j < currentRow.length; j++) {
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sum += currentRow[j];
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}
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}
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return sum;
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}
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```
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While there are some scenarios where it may be fitting to take advantage of it, you should use your best judgement.
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# `const` declarations
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`const` declarations are another way of declaring variables.
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```typescript
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const numLivesForCat = 9;
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```
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They are like `let` declarations but, as their name implies, their value cannot be changed once they are bound.
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In other words, they have the same scoping rules as `let`, but you can't re-assign to them.
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### ~button /js/operators
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NEXT: Operators
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### ~ |