用函数式的方式思考——递归

function fact(n: number): number {  let result = 1;  for (let i = 0; i <= n; i += 1) {    result *= i;  }  return result;}

Model -> View

View = f(Model)

function exp(a: number, n: number) {  return a * exp(a, n - 1);}

function expImpl(a: number, prev: number): number;

function expImpl(a: number, prev: number, i: number, n: number): number;

function expImpl(a: number, prev: number, i: number, n: number): number {  if (i >= n) {    return prev;      }  return expImpl(a, a * prev, i + 1, n);}
function exp(a: number, n: number) {  return expImpl(a, 1, 0, n);}

function expImpl(a: number, prev: number, i: number) {  if (i <= 0) {    return prev;  }  return expImpl(a, a * prev, i - 1);}
function exp(a: number, n: number) {  return expImpl(a, 1, n);}

function exp(a: number, n: number) {  let i = n;  let ret = 1;  while (i > 0) {    ret = ret * a;    i--;  }  return ret;}

function fastExpImpl(a: number, prev: number, i: number) {  if (i <= 0) {    return prev;  }  if (i % 2 === 0) {    return expImpl(a * a, prev, i / 2);  }  // 这里还可以做个小优化 expImpl(a * a, a * prev, (i - 1) / 2);  return expImpl(a, a * prev, i - 1);}
function fastExp(a: number, n: number) {  return expImpl(a, 1, n);}

function fib(n: number): number {  if (n <= 2) {    return 1;  }  return fib(n - 1) + fib(n - 2);}

a[n - 2] + a[n - 1] -> a[n]

function fibImpl(a: number, b: number): number;

function fibImpl(a: number, b: number, i: number, n: number): number;

function fibImpl(a: number, b: number, i: number): number {  if (i <= 1) {    return a;  }  return fibImpl(a + b, a, i - 1);}
function fib(n: number): number {  return fibImpl(1, 0, n);}

function fib(n: number): number {  let a = 1;  let b = 0;  let i = n;  while (i > 1) {    a = a + b;    b = a;    i--;  }  return a;}

a <- a + bb <- a