@material-ui/core/es/styles/createBreakpoints.js

import _extends from "@babel/runtime/helpers/esm/extends";
import _objectWithoutPropertiesLoose from "@babel/runtime/helpers/esm/objectWithoutPropertiesLoose";
// Sorted ASC by size. That's important.
// It can't be configured as it's used statically for propTypes.
export const keys = ['xs', 'sm', 'md', 'lg', 'xl']; // Keep in mind that @media is inclusive by the CSS specification.

export default function createBreakpoints(breakpoints) {
  const {
    // The breakpoint **start** at this value.
    // For instance with the first breakpoint xs: [xs, sm[.
    values = {
      xs: 0,
      sm: 600,
      md: 960,
      lg: 1280,
      xl: 1920
    },
    unit = 'px',
    step = 5
  } = breakpoints,
        other = _objectWithoutPropertiesLoose(breakpoints, ["values", "unit", "step"]);

  function up(key) {
    const value = typeof values[key] === 'number' ? values[key] : key;
    return `@media (min-width:${value}${unit})`;
  }

  function down(key) {
    const endIndex = keys.indexOf(key) + 1;
    const upperbound = values[keys[endIndex]];

    if (endIndex === keys.length) {
      // xl down applies to all sizes
      return up('xs');
    }

    const value = typeof upperbound === 'number' && endIndex > 0 ? upperbound : key;
    return `@media (max-width:${value - step / 100}${unit})`;
  }

  function between(start, end) {
    const endIndex = keys.indexOf(end);

    if (endIndex === keys.length - 1) {
      return up(start);
    }

    return `@media (min-width:${typeof values[start] === 'number' ? values[start] : start}${unit}) and ` + `(max-width:${(endIndex !== -1 && typeof values[keys[endIndex + 1]] === 'number' ? values[keys[endIndex + 1]] : end) - step / 100}${unit})`;
  }

  function only(key) {
    return between(key, key);
  }

  let warnedOnce = false;

  function width(key) {
    if (process.env.NODE_ENV !== 'production') {
      if (!warnedOnce) {
        warnedOnce = true;
        console.warn(["Material-UI: The `theme.breakpoints.width` utility is deprecated because it's redundant.", 'Use the `theme.breakpoints.values` instead.'].join('\n'));
      }
    }

    return values[key];
  }

  return _extends({
    keys,
    values,
    up,
    down,
    between,
    only,
    width
  }, other);
}