Pushing array into an array of array in TS

I am coming from Python, and am not entirely familiar with the ‘proper’ JS/TS way of doing things.

I am looping through the elements of a set, and I am pushing lists of some of the elements onto a 2D array.

let res: number[][];
for (let posElement of posSet) {
    if (negSet.has(-1*posElement)) {
        res.push([-1*posElement, 0, posElement]);
    }
}

I am getting the following error TypeError: Cannot read properties of undefined (reading 'push'). What am I doing wrong, where am I messing up the syntax?

Answer

like this initialize

let res: number[][]=[];