Typescript : Trying the addition of two variables but get the concatenation of the two

Question

I have three variable in my Typescript class :

A:number; B:number; C:number; 

in another part of the class i try to make the addition of the two variable A and B :

this.C = this.A+this.B; // A =20 and B = 50; 

and I display C in the html template

<span>{{C}}</span> 

My problem is, instead of getting the addition of the TWO variable (20+50=70) i get the concatenation (2050)!!

Can someone help me please ?

UPDATE :

Here is the exact code portion that cause problem :

goTo(page:number,type:script) {     //         this.pageFirstLineNumber = page;     this.pageLastLineNumber = page + this.LINE_OFFSET; //concatenation!! } 

Notice that pageLastNumber is declared as number type, LINE_OFFSET is olso number type, i have found a solution to this issue but the typescript compiler output an error (forbidden eval):

//// .... this.pageFirstLineNumber = eval(page.toString()); // now It works !! this.pageLastLineNumber = page + this.LINE_OFFSET; //concatenation!! 

UPDATE

Here is the declaration of the LINE_OFFSET variable :

private _calculateOffset(fontSize: number) {     let linesDiff = (fontSize * 27) / 14;     let lines:number = 27 - (linesDiff - 27);     this.LINE_OFFSET = Math.floor(lines);     if (fontSize >= 17 && fontSize <= 20) {         this.LINE_OFFSET += (Math.floor(fontSize / 3) - 2);     }     if (fontSize > 20 && fontSize <= 23) {         this.LINE_OFFSET += (Math.floor(fontSize / 2) - 2);     }     if (fontSize > 23 && fontSize <= 25) {         this.LINE_OFFSET += (Math.floor(fontSize / 2));}     if (fontSize > 25 && fontSize <= 27) {         this.LINE_OFFSET += (Math.floor(fontSize / 2) + 1);     }     if (fontSize > 27 && fontSize <= 30) {         this.LINE_OFFSET += (Math.floor(fontSize / 2) + 4);     } } 

Answer 1

prepend the numbers with +:

let a = +b + +c; 

ref

Answer 2

When you declare in an interface that a property is a number then it stays as a declaration only, it won't be translated into javascript.

For example:

interface Response {     a: number;     b: number; }  let jsonString = '{"a":"1","b":"2"}'; let response1 = JSON.parse(jsonString) as Response;  console.log(typeof response1.a); // string  console.log(typeof response1.b); // string console.log(response1.a + response1.b); // 12 

As you can see, the json has the a and b as strings and not as numbers and declaring them as numbers in the interface has no effect on the runtime result.

If what you get from your server is encoded as strings instead of numbers then you'll need to convert them, for example:

let response2 = {     a: Number(response1.a),     b: Number(response1.b) } as Response;  console.log(typeof response2.a); // number  console.log(typeof response2.b); // number console.log(response2.a + response2.b); // 3 

(entire code in playground)