Solution.js

/**
 * @param {number[][]} matrix
 */
var NumMatrix = function (matrix) {
    const m = matrix.length;
    const n = matrix[0].length;
    this.s = new Array(m + 1).fill(0).map(() => new Array(n + 1).fill(0));
    for (let i = 0; i < m; ++i) {
        for (let j = 0; j < n; ++j) {
            this.s[i + 1][j + 1] =
                this.s[i + 1][j] + this.s[i][j + 1] - this.s[i][j] + matrix[i][j];
        }
    }
};

/**
 * @param {number} row1
 * @param {number} col1
 * @param {number} row2
 * @param {number} col2
 * @return {number}
 */
NumMatrix.prototype.sumRegion = function (row1, col1, row2, col2) {
    return (
        this.s[row2 + 1][col2 + 1] -
        this.s[row2 + 1][col1] -
        this.s[row1][col2 + 1] +
        this.s[row1][col1]
    );
};

/**
 * Your NumMatrix object will be instantiated and called as such:
 * var obj = new NumMatrix(matrix)
 * var param_1 = obj.sumRegion(row1,col1,row2,col2)
 */