Skip to main content
Engineering LibreTexts

7.9: JavaScript Examples

  • Page ID
    10708
  • Strings

    // This program demonstrates string functions.
    
    main();
    
    function main()
    {
        var str = "Hello";
    
        output("string: " + str);
        output("string.toLowerCase(): " + str.toLowerCase());
        output("string.toUpperCase(): " + str.toUpperCase());
        output("string.indexOf('e'): " + str.indexOf('e'));
        output("string.length: " + str.length);
        output("string.replace('H', 'j'): " + str.replace('H', 'j'));
        output("string(substring(2,4): " + str.substring(2, 4));
        output("string.trim(): " + str.trim());
    
        var name = "Bob";
        var value = 123.456;
        output(`string.format(): ${name} earned $${value.toFixed(2)}`);
    }
    
    // Checks the JavaScript environment and writes to the console, 
    // the current document, or standard output as appropriate.
    // Reference: http://progopedia.com/example/hello-world/114/ 
    function output(text) {
        if (typeof console === "object") {
          console.log(text);
        } 
        else if (typeof document === "object") {
          document.write(text);
        } 
        else {
          print(text);
        }
    }
    

    Output

    string: Hello
    string..toLowerCase(): hello
    string.toUpperCase(): HELLO
    string.indexOf('e'): 1
    string.length: 5
    string.replace('H', 'j'): jello
    string(substring(2,4): ll
    string.trim(): Hello
    string.format(): Bob earned $123.46
    

    Files

    Note: For security reasons, JavaScript in a browser requires the user to select the file to be processed. This example is based on node.js rather than browser-based JavaScript.

    // This program creates a file, adds data to the file, displays the file,
    // appends more data to the file, displays the file, and then deletes the file.
    // It will not run if the file already exists.
    
    function calculateFahrenheit(celsius) {
        fahrenheit = celsius * 9 / 5 + 32
        return fahrenheit
    }
    
    function createFile(filename) {
        var fs = require('fs')
    
        fs.writeFile(filename, "Celsius,Fahrenheit\n",  function(err) {
            if (err) return console.error(err);
        });
    
        for(var celsius = 0; celsius <= 50; celsius++) {
            var fahrenheit = calculateFahrenheit(celsius);
            fs.appendFile(filename, celsius.toFixed(1) + "," + 
                fahrenheit.toFixed(1) + "\n", function (err) {
                if (err) {
                    return console.error(err);
                }
            });
        }
    }
    
    function readFile(filename) {
        var file = require('readline').createInterface( {
          input: require('fs').createReadStream(filename)
        });
        
        file.on('line', function (line) {
            console.log(line);
        });
    }
    
    function appendFile(filename) {
        var fs = require('fs')
    
        for(var celsius = 51; celsius <= 100; celsius++) {
            var fahrenheit = calculateFahrenheit(celsius);
            fs.appendFile(filename, celsius.toFixed(1) + "," + 
                fahrenheit.toFixed(1) + "\n", function (err) {
                if (err) {
                    return console.error(err);
                }
            });
        }
    }
    
    function deleteFile(filename) {
        var fs = require("fs");
    
        fs.unlink(filename, function(err) {
            if (err) {
                return console.error(err);
            }
        });
    }
    
    function fileExists(filename) {
        var fs = require('fs');
        return fs.existsSync(filename);
    }
    
    function main() {
        var filename = "~file.txt";
    
        if(fileExists(filename)) {
            console.log("File already exists.")
        } else {
            createFile(filename);
            readFile(filename);
            appendFile(filename);
            readFile(filename);
            deleteFile(filename);
        }
    }
    
    main();
    

    Output

    Celsius,Fahrenheit
    0.0,32.0
    1.0,33.8
    2.0,35.6
    ...
    98.0,208.4
    99.0,210.2
    100.0,212.0