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JavaScript | async function expression
  • Last Updated : 03 Jun, 2020

Async function expression is used to define an async function inside an expression in JavaScript. The async function is declared using the async keyword.

Syntax:

async function [function_name]([param1[, param2[, ..., paramN]]]) {
    // Statements
}

Parameters:

  • function_name: This parameter holds the function name. This function name is local to the function body. If function name is ommitted then it becomes anonymous function.
  • paramN: It is the name of parameter that to be passed into the function.
  • Statements: It contains the body of the function.

Return Value: It returns a promise to return the value or else throw an exception, whenever an error occurs.

Example 1: In this example “GeeksforGeeks” is printed first and after an interval of 1000 ms, “GFG” is printed.



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<script>
    function cb() {
        return new Promise(function (resolve, reject) {
            setTimeout(function () {
                resolve("GFG")
            }, 1000)
  
        })
    }
    async function course() {
        console.log("GeeksforGeeks");
        const result = await cb();
        console.log(result);
    }
  
    course();
</script>

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Output:

GeeksforGeeks
GFG

Example 2: Here, a file is made gfg.txt and as soon as the file is read it prints “Read the file” in the console. Else it prints “error” when either the location of the file is wrong or it is not unable to read the file due to any other reason.

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<script>
    async function gfg() {
        try {
            let f1 = await fs.promises.readFile("gfg.txt")
            console.log("Read the file")
        }
        catch (err) {
            console.log("error");
        }
    }
    gfg();
</script>

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Output:

  • When file read:
    Read the file
  • When file is not read(error thrown)
    error

Example 3: This is an example of async function working in parallel.

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<script>
    function cb() {
        return new Promise(function (resolve, reject) {
            setTimeout(function () {
                resolve("GFG")
            }, 2000)
  
        })
    }
  
    function cb1() {
        return new Promise(function (resolve, reject) {
            setTimeout(function () {
                resolve("GFG1")
            }, 1000)
  
        })
    }
  
    async function course() {
        console.log("GeeksforGeeks");
        const result1 = await cb1();
        const result = await cb();
        console.log(result1);
        console.log(result);
    }
  
    course();
</script>

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Output:

GeeksforGeeks
GFG1
GFG

Supported Browsers:

  • Google Chrome
  • Edge
  • Firefox
  • Safari
  • Opera

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