如何使用''let“代替”var“

Question

Let's say I have this code:

  if (input.length >= 3) {
     var filtered_school = _.filter(schools, function (school) {
         return _.startsWith(school.label, input);
    });
  }
this.setState({ options: filtered_school })

I can't use 'let' as it can't be seen outside the scope so I can't assign filtered_school to options .

But I hear that there is no reason to use var in ES6.

How can I use let in this situation?

Answer 1

Just put it outside the if :

let filtered_school;
if (input.length >= 3) {
  filtered_school = // ...
});

let is block-scoped, meaning if you define it within the if block, it won't exist outside of it so you have to extract it out in this situation.

Answer 2

You declare the variable in the scope you want to use it in. ie outside of the if block.

let filtered_school;
if (input.length >= 3) {
  filtered_school = _.filter(schools, function(school) {
    return _.startsWith(school.label, input);
  });
}
this.setState({
  options: filtered_school
})

Answer 3

let creates block level scope in ES6, you can declare it outside and assign it in your filter .

let filtered_school;
 if (input.length >= 3) {
    filtered_school = _.filter(schools, function (school) {
         return _.startsWith(school.label, input);
    });
  }
this.setState({ options: filtered_school })

Answer 4

let is block scoped, so if let is somewhere inside {} , or in a logical block, it will only be accessible there. To make it accessibly outside of your example place it outside the if statement.

 let filtered_school;
 if (input.length >= 3) {
     filtered_school = _.filter(schools, function (school) {
         return _.startsWith(school.label, input);
    });
 }