Pass only the second argument in javascript

Question

I'm trying to make a function in which I pass only the second argument of my function.

I would like it to work this way:

function test (a,b) {      // ... }; // pass only the second parameter  test( ... , b); 

My current idea is to pass the second argument as a de facto dynamic default parameter as following:

var defaultVar = "something";      function test (a, b=defaultVar) {     // ... } 

...then change the defaultVar value according to my needs.

var defaultVar = modification;  

In fact, I'm using the Google drive API, and I'm trying to make it such that I can enter a string value for the second parameter to make a callback. This callback would take the role of verifying whether the return file is effectively the file searched (by making a boolean verification on name value).

Hence, the idea to me is to automate the process of getting a file on Google drive by passing his name and retrieving the file data in this way.

I hope this precision will be useful.

Here is my quickstart.js :

// (...Google authentication and all) ;   var filename = ""; // enter a filename in the function by the way of filename function listFiles (auth, filename =  filename) {   const drive = google.drive({version: 'v3', auth});   drive.files.list({     pageSize: 50,     fields: 'nextPageToken, files(id, name)',   }, (err, {data}) => {     if (err) return console.log('The API returned an error: ' + err);     const files = data.files;     if (files.length) {       console.log('Files:');       files.map((file) => {         console.log(`${file.name} (${file.id})`);          // check if the file returns match the filename wished          displayFile(file);         if(`${file.name}` == filename ){           console.log("name found !");           const fileData = {             name : `${file.name}`,             id : `${file.id}`           };           return fileData;         }       });     } else {       console.log('No files found.');     }   }); }  listFiles(undefined, "test.md") 

Any improving ideas are welcome.

Answer 1

With default parameter values added in ES2015, you can declare default values for the parameters, and when making the call, if you pass undefined as the first parameter, it will get the default:

function test(a = "ay", b = "bee") {    console.log(`a = ${a}, b = ${b}`);  }  test();             // "a = ay, b = bee"  test(1);            // "a = 1, b = bee"  test(undefined, 2); // "a = ay, b = 2"  test(1, 2);         // "a = 1, b = 2"

You can do something similar yourself manually in a pre-ES2015 environment by testing for undefined:

function test(a, b) {    if (a === undefined) {      a = "ay";    }    if (b === undefined) {      b = "bee";    }    console.log("a = " + a + ", b = " + b);  }  test();             // "a = ay, b = bee"  test(1);            // "a = 1, b = bee"  test(undefined, 2); // "a = ay, b = 2"  test(1, 2);         // "a = 1, b = 2"

Answer 2

If you are OK with a change to the number of arguments that your function takes, you could pass the arguments via object properties. Then you can let the caller decide which property (or properties) to specify during the call.

The other properties can take default values through object destructuring in the function's parameter specification:

function test({a = 1, b = 2}) {      console.log(`a = ${a}, b = ${b}`);  };    test({b:42}); // only specify what b is