Variables are explicitly declared and used by the compiler to check that they are used as expected for their type. You don’t have to declare the type if the compiler can infer it from its use.
let a = "hello";
console.log(a, typeof a);
let b = 1;
console.log(b, typeof b);
let c = b;
console.log(c, typeof c);
// error: c is a number, can't assign a string to it
// c = a;
let d = true;
console.log(d, typeof d);
let e: number;
// error: can't use e before it's assigned a value
// console.log(e, typeof(e));
e = 4;
console.log(e, typeof e);
hello string
1 number
1 number
true boolean
4 number
Note
Always uselet to declare a variable unless you know the value is
constant, which we’ll discuss in the next section.Warning: let vs. var
Although TypeScript allows using var to declare variables, don’t use it –
always use let or const instead.
var is available to support legacy behavior from older JavaScript, but it’s
use is problematic in modern TypeScript. See the
Handbook
to dig deeper.