This is the second day of my participation in the November Gwen Challenge. Check out the details: the last Gwen Challenge 2021

Block-level scope

The ES2015 standard has added block-scoping. In this article, we will look at what block-level scopes are.

Variables declared through var or function declarations created in non-strict mode have no block-level scope. Variables declared in a block are not only scoped within the function or script tag they are in; the effects of the set variable persist beyond the block itself.

Block-level scope is used to combine zero or more statements, and the block is defined by a pair of braces.

How to use

  1. varThe statement
var name = 'also laugh';
{
  var name = 'slifree';
}
console.log(name); // slifree
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Var declares variables that have no block-level scope.

  1. letThe statement
let name = 'also laugh';
{
  let name = 'slifree';
}
console.log(name); / / also smiled
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Variables declared by let are block-scoped, and only variables of the current scope can be retrieved. If two identical variables are declared by let in the same scope, an error will be reported as follows:

SyntaxError: Identifier 'name' has already been declared
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If we wrap a separate scope with {} as in the code above, no errors are reported.

  1. constThe statement
let const = 'also laugh';
{
  let const = 'slifree';
}
console.log(const); / / also smiled
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The use of const is similar to let, but it is different from let, and this is where we need to pay attention.

Matters needing attention

Let declares a variable whose value can be changed; A variable declared by const, whose value generally cannot be changed. Const is a constant and cannot be changed if the data is of an underlying type, as follows:

const name = 'also laugh';
name = 'slifree';
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In this case, an error message is displayed as follows:

TypeError: Assignment to constant variable.
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If the data is of reference type, it may be changed as follows:

Do not use it this way, otherwise an error will be reported

const obj = {};
obj = { name: 'also laugh' }
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You can change the value in this way

const obj = {};
obj.name = 'also laugh'
console.log(obj); // {name: 'also laugh'}
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conclusion

  1. In the project, the variables we define have a specific meaning and are not allowed to be defined repeatedly, so we generally useletDeclare variables.
  2. constIt is used to declare a constant, meaning that the value of a variable we define will not change.

If you want to learn more about ES features, go to ES2015+ Fragmentation Learning, taking a step forward every day, which is the directory entry for all the articles.