[TypeScript] Collect Related Strings in a String Enum in TypeScript

Posted 2021-01-06 answer1215

As of TypeScript 2.4, it is now possible to define string enums, or more precisely, enums with string members. Just like any other numeric enum, string enums can be made constant using the const modifier so that they disappear entirely from the generated javascript; in this case, all enum values will be inlined into the output.

 

For example,we  have the code:

fetch("https://swapi.co/api/people/1/", {
  headers: {
      Accept: ‘application/json‘
  }
})
.then((res) => res.json())
.then(response => {
    console.log(response.name)
});

 

We want to replace ‘application/json‘ to use Typescript enum.

enum MediaTypes {
  JSON = "application/json"
}

fetch("https://swapi.co/api/people/1/", {
  headers: {
      Accept: MediaTypes.JSON
  }
})
.then((res) => res.json())
.then(response => {
 console.log(response.name)
});

 

From the compiled code, we can see the output:

var MediaTypes;
(function (MediaTypes) {
    MediaTypes["JSON"] = "application/json";
})(MediaTypes || (MediaTypes = {}));
fetch("https://swapi.co/api/people/1/", {
    headers: {
        Accept: MediaTypes.JSON
    }
})
    .then(function (res) { return res.json(); })
    .then(function (response) {
    console.log(response.name);
});

The compile code, it add a IIFE define and set JSON code to ‘application/json‘.

 

Sometime, we don‘t want this meta code goes into production code, the way to do this is add "const":

const enum MediaTypes {
  JSON = "application/json"
}
/*compiled code**/

fetch("https://swapi.co/api/people/1/", {
    headers: {
        Accept: "application/json" /* JSON */
    }
})
    .then(function (res) { return res.json(); })
    .then(function (response) {
    console.log(response.name);
});

As we can see, the output code doesn‘t have IIFE anymore, the code is much smaller.

 

You can get reverse mapping by using number:

enum Port { 
 NUM = 412
}

/**compiled code*/
(function (Port) {
    Port[Port["NUM"] = 412] = "NUM";
})(Port || (Port = {}));

 

Last thing, if you really want to use "const" keyword but still want to keep IIFE meta code, you can set up in tsconfig.json:

{
  "preserveConstEnums": true
}