async関数の作成、try/catch、並列処理と逐次処理

async/awaitの概要

小さなPromiseヘルパー

// delay returns a Promise that resolves after ms
function delay(ms, value) {
  return new Promise(function (resolve) {
    setTimeout(function () { resolve(value); }, ms);
  });
}

// Example usage
delay(10, "ok").then(function (v) {
  console.log("delay:", v);
});

async関数の基本

// An async function can use await inside
async function greet() {
  const name = await delay(5, "Ayla");
  console.log("Hello,", name);
}

// Call it
greet();

awaitでのtry/catch

// Simulate a failing task
function failAfter(ms, message) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () { reject(new Error(message)); }, ms);
  });
}

async function runSafe() {
  try {
    const v = await failAfter(5, "boom");
    console.log("Value:", v);
  } catch (e) {
    console.log("Caught:", e.message);
  }
}

runSafe();

順次実行のパターン

// Sequential: total time ~ A + B
async function sequential() {
  const a = await delay(8, "A");
  const b = await delay(8, "B");
  console.log("seq:", a + b);
}

sequential();

allによる並列実行

// Parallel: start both, then await together
async function parallel() {
  const pa = delay(8, "A");
  const pb = delay(8, "B");
  const results = await Promise.all([pa, pb]);
  console.log("par:", results.join(""));
}

parallel();

並列実行と順次実行のクイズ

振り返り