Mastering TypeScript 3 - Third Edition

Default parameters

A subtle variant of the optional parameter syntax allows us to specify the default value of a parameter, if it is not supplied. Let's modify our preceding function definition to use an optional parameter with a default value, as follows:

function concatStringsDefault(  
  a: string,  
  b: string,  
  c: string = "c") { 
    return a + b + c; 
} 
var defaultConcat = concatStringsDefault("a", "b"); 
console.log(`defaultConcat : ${defaultConcat}`); 

This function definition has now dropped the ? optional parameter syntax, but instead has assigned a value of "c" to the last parameter, c:string = "c". By using default parameters, if we do not supply a value for the parameter named c, the concatStringsDefault function will substitute the default ...