TypeScript implementation of simple SymmetricMatrix

SymmetricMatrix.ts

export class SymmetricMatrix<T> {
    protected data: T[];

    constructor(
        public readonly size: number,
        initializer: (i: number, j: number) => T,
    ) {
        this.data = new Array(size);

        for (let i = 0; i < size; ++i) {
            for (let j = 0; j <= i; ++j) {
                this.set(i, j, initializer(i, j));
            }
        }
    }

    // i-th row, j-th column
    set(i: number, j: number, value: T) {
        return (this.data[this.get1DIndex(i, j)] = value);
    }

    get(i: number, j: number) {
        return this.data[this.get1DIndex(i, j)];
    }
  
    private get1DIndex(i: number, j: number) {
        if (i < j) {
            const temp = i;
            i = j;
            j = temp;
        }

        return (i * (i + 1)) / 2 + j;
    }

    [Symbol.iterator]() {
        return this.data[Symbol.iterator]();
    }
    
    /**
     * Iterate every elements with rich indices
     * Abort if returned value is false
     * @param callback
     */
    forEach(callback: (value: T, i: number, j: number) => boolean | undefined) {
        for (let i = 0; i < this.size; ++i) {
            for (let j = 0; j <= i; ++j) {
                if (callback(this.get(i, j), i, j) === false) return;
            }
        }
    }
}

This is an implementation for symmetric matrix for arbitrary type T. This only uses N * (N - 1) / 2 spaces. Also this doesn't use 2D array so that less overhead exists.

The index of 2D coordinate is computed by following manner, where i is index of row and j is index of column with zero based.

0
1        2
3        4        5
...
∑i      (∑i)+1  ...  (∑i)+j