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