typescript didn't report any error when function may return undefined

Question

getStage may return undefined but didn't report any error.it will make my code crash.

    const a = Math.random() > 0.4
    function getStage(): string {
        if(a) {
            return ''
        }
    }
    const str: string = getStage()

after tsc compiled

    var a = Math.random() > 0.4;
    function getStage() {
        if (a) {
            return '';
        }
    }
    var str = getStage();

Answer 1

strictNullChecks - check if it off in your ts config