TypeScript – Variables, Array and Methods

By | 07/04/2021

In this post, we will see what TypeScript is and how to configure our system, in order to use it for developing application.
First of all, what is TypeScript?
From Wikipedia:
TypeScript is a programming language developed and maintained by Microsoft. It is a strict syntactical superset of JavaScript and adds optional static typing to the language. TypeScript is designed for the development of large applications and transcompiles to JavaScript.
As TypeScript is a superset of JavaScript, existing JavaScript programs are also valid TypeScript programs.

In simple terms, TypeScript extends JavaScript by adding data types, classes, and other object-oriented features with type-checking. It is a typed superset of JavaScript that compiles to plain JavaScript.
For all information, go to the official web site: https://www.typescriptlang.org

In order to use TypeScript, we need the compiler that we can install using the command
npm install -g typescript.
Now, we open Visual Studio Code and we start to use TypeScript.

The first thing to do, it is to create the project config file using the command tsc –init:

Then, we create two directories: one called src (where we will create the files .ts) and one called public (where we will save the javascript files):

Finally, we have to modify the config file, in order to use these two directories:

{
  "compilerOptions": {
    /* Visit https://aka.ms/tsconfig.json to read more about this file */

    /* Basic Options */
    // "incremental": true,                   /* Enable incremental compilation */
    "target": "es6",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019', 'ES2020', or 'ESNEXT'. */
    "module": "es2015",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', 'es2020', or 'ESNext'. */
    // "lib": [],                             /* Specify library files to be included in the compilation. */
    // "allowJs": true,                       /* Allow javascript files to be compiled. */
    // "checkJs": true,                       /* Report errors in .js files. */
    // "jsx": "preserve",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
    // "declaration": true,                   /* Generates corresponding '.d.ts' file. */
    // "declarationMap": true,                /* Generates a sourcemap for each corresponding '.d.ts' file. */
    // "sourceMap": true,                     /* Generates corresponding '.map' file. */
    // "outFile": "./",                       /* Concatenate and emit output to single file. */
    "outDir": "./public",                     /* Redirect output structure to the directory. */
    "rootDir": "./src",                        /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
    // "composite": true,                     /* Enable project compilation */
    // "tsBuildInfoFile": "./",               /* Specify file to store incremental compilation information */
    // "removeComments": true,                /* Do not emit comments to output. */
    // "noEmit": true,                        /* Do not emit outputs. */
    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
    // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
    // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */

    /* Strict Type-Checking Options */
    "strict": true,                           /* Enable all strict type-checking options. */
    // "noImplicitAny": true,                 /* Raise error on expressions and declarations with an implied 'any' type. */
    // "strictNullChecks": true,              /* Enable strict null checks. */
    // "strictFunctionTypes": true,           /* Enable strict checking of function types. */
    // "strictBindCallApply": true,           /* Enable strict 'bind', 'call', and 'apply' methods on functions. */
    // "strictPropertyInitialization": true,  /* Enable strict checking of property initialization in classes. */
    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */

    /* Additional Checks */
    // "noUnusedLocals": true,                /* Report errors on unused locals. */
    // "noUnusedParameters": true,            /* Report errors on unused parameters. */
    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
    // "noUncheckedIndexedAccess": true,      /* Include 'undefined' in index signature results */

    /* Module Resolution Options */
    // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
    // "paths": {},                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
    // "typeRoots": [],                       /* List of folders to include type definitions from. */
    // "types": [],                           /* Type declaration files to be included in compilation. */
    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
    "esModuleInterop": true,                  /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */
    // "allowUmdGlobalAccess": true,          /* Allow accessing UMD globals from modules. */

    /* Source Map Options */
    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */

    /* Experimental Options */
    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */

    /* Advanced Options */
    "skipLibCheck": true,                     /* Skip type checking of declaration files. */
    "forceConsistentCasingInFileNames": true  /* Disallow inconsistently-cased references to the same file. */
  },
  /* we will include only the 'src' folder */
  "include": ["src"]
}



Now we are ready to use TypeScript!

VARIABLES

[CORE.TS]

// Type of variables

// bool
let valueBool: boolean = true;
console.log(valueBool)

// number
let valueNumber: number = 100;
console.log(valueNumber);

// string
let valueString: string = 'Hello World';
console.log(valueString);


// number and string
let valueMix: string | number;
valueMix = 1;
console.log(`The first value of valueMix is: ${valueMix}`);
valueMix = "one"
console.log(`The second value of valueMix is: ${valueMix}`);


// any
let valueAny: any;
valueAny = 2;
console.log(`The first value of valueAny is: ${valueAny}`);
valueAny = 'two';
console.log(`The second value of valueAny is: ${valueAny}`);
valueAny = true;
console.log(`The third value of valueAny is: ${valueAny}`);



Now, with the command tsc, we will compile the file .tsc and the output will be a file called core.js:

[CORE.JS]

"use strict";
// Type of variables
// bool
let valueBool = true;
console.log(valueBool);
// number
let valueNumber = 100;
console.log(valueNumber);
// string
let valueString = 'Hello World';
console.log(valueString);
// number and string
let valueMix;
valueMix = 1;
console.log(`The first value of valueMix is: ${valueMix}`);
valueMix = "one";
console.log(`The second value of valueMix is: ${valueMix}`);
// any
let valueAny;
valueAny = 2;
console.log(`The first value of valueAny is: ${valueAny}`);
valueAny = 'two';
console.log(`The second value of valueAny is: ${valueAny}`);
valueAny = true;
console.log(`The third value of valueAny is: ${valueAny}`);



Finally, with the command node core.js, we will run the script:

ARRAY

[CORE.TS]

// Array of string
let lstString: string[] = [];
lstString.push('One');
lstString.push('Two');
lstString.push('Three');

lstString.forEach(item => {
    console.log(item);
});


// Array of string & number
let lstMix: (string|number)[] = [];
lstMix.push('One');
lstMix.push(2);
lstMix.push('Three');
lstMix.push(4);

lstMix.forEach(item => {
    console.log(item);
});


// Array of any
let lstAny: any[] = [];
lstAny.push(1);
lstAny.push('Two');
lstAny.push(true);

lstAny.forEach(item => {
    console.log(item);
});



Now, if we run this script, this will be the result:

METHODS

[CORE.TS]

// Method void
function Method1(): void{
    console.log('Method1');
}

function Method2(){
    console.log('Method2');
}

// arrow function
let Method3 = (): void =>{
    console.log('Method3')
}

let Method4 = () =>{
    console.log('Method4')
}


Method1();
Method2();
Method3();
Method4();



// Method with value in output
function Method1A(): string{
    return 'Method1A';
}

// arrow function
let Method2A = (): string =>{
    return 'Method2A';
}

console.log(Method1A());
console.log(Method2A());



// Method with parameters in input
function MethodPara1(val1: number, val2: number):number{
    return val1 + val2;
}
console.log(`MethodPara1(5,5): ${MethodPara1(5,5)}`)

// parameter with a default value
function MethodPara2(val1: number, val2: number = 8):number{
    return val1 + val2;
}
console.log(`MethodPara2(10,10): ${MethodPara2(10,10)}`)
console.log(`MethodPara2(20): ${MethodPara2(20)}`)



// Function Signatures
let MethodSignature: Function;

MethodSignature = (): void =>{
    console.log('MethodSignature without parameters');
}
MethodSignature();

MethodSignature = (val1: number): number =>{
    return val1*val1;
}

console.log(`MethodSignature with parameter: ${MethodSignature(5)}`);



Now, If we run this script, this will be the result:



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