Math.floor

Math.floor(x)

Math.floor( 45.95 //  45
Math.floor( 45.05 //  45
Math.floor(  4    //   4
Math.floor(-45.05 // -46 
Math.floor(-45.95 // -46

// Closure
(function() {
  /**
   * Decimal adjustment of a number.
   *
   * @param {String}  type  The type of adjustment.
   * @param {Number}  value The number.
   * @param {Integer} exp   The exponent (the 10 logarithm of the adjustment base).
   * @returns {Number} The adjusted value.
   */
  function decimalAdjust(type, value, exp) {
    // If the exp is undefined or zero...
    if (typeof exp === 'undefined' || +exp === 0) {
      return Math[type](value
    }
    value = +value;
    exp = +exp;
    // If the value is not a number or the exp is not an integer...
    if (isNaN(value) || !(typeof exp === 'number' && exp % 1 === 0)) {
      return NaN;
    }
    // Shift
    value = value.toString().split('e'
    value = Math[type](+(value[0] + 'e' + (value[1] ? (+value[1] - exp) : -exp))
    // Shift back
    value = value.toString().split('e'
    return +(value[0] + 'e' + (value[1] ? (+value[1] + exp) : exp)
  }

  // Decimal round
  if (!Math.round10) {
    Math.round10 = function(value, exp) {
      return decimalAdjust('round', value, exp
    };
  }
  // Decimal floor
  if (!Math.floor10) {
    Math.floor10 = function(value, exp) {
      return decimalAdjust('floor', value, exp
    };
  }
  // Decimal ceil
  if (!Math.ceil10) {
    Math.ceil10 = function(value, exp) {
      return decimalAdjust('ceil', value, exp
    };
  }
})(

// Round
Math.round10(55.55, -1   // 55.6
Math.round10(55.549, -1  // 55.5
Math.round10(55, 1       // 60
Math.round10(54.9, 1     // 50
Math.round10(-55.55, -1  // -55.5
Math.round10(-55.551, -1 // -55.6
Math.round10(-55, 1      // -50
Math.round10(-55.1, 1    // -60
// Floor
Math.floor10(55.59, -1   // 55.5
Math.floor10(59, 1       // 50
Math.floor10(-55.51, -1  // -55.6
Math.floor10(-51, 1      // -60
// Ceil
Math.ceil10(55.51, -1    // 55.6
Math.ceil10(51, 1        // 60
Math.ceil10(-55.59, -1   // -55.5
Math.ceil10(-59, 1       // -50