Handling Errors in Asynchronous Functions with JavaScriptLet's explore the best practices for

Question

Handling Errors in Asynchronous Functions with JavaScriptLet's explore the best practices for

I am a beginner in javascript and recently delved into async/await. After going through various resources, I gained a basic understanding. However, while experimenting with some code examples, I encountered unexpected results which left me puzzled about where I went wrong.

Code:

var colors = ["RED", "GREEN", "YELLOW"];

const getColor = async () => {
  var value = "";
  colors.forEach((color) => {
    value = value + color + " ";
  });
  return value;
};


const middleware = async () => {
  addColor(null)
    .then(() => {
      getColor().then((result) => {
        console.log(result);
      });
    })
    .catch((err) => {
      console.log(err.message + " at middleware");
    });
};

const addColor = async (color) => {
  validateColor(color)
    .then(() => {
      console.log("Adding data");
      colors.push(color);
    })
    .catch((err) => {
      console.log(err.message + " at add color");
      throw err;
    });
};

const validateColor = async (color) => {
  if (color == null) {
    throw new Error("Color cannot be empty");
  }
};

middleware();

Upon executing the middleware function, instead of only displaying the error message as expected, the output included the names of colors as well. Output:

https://i.sstatic.net/YfIPM.png

I am perplexed as to why the code inside then() is being executed even when addColor() throws an error. Furthermore, it's puzzling why the catch block within middleware() is not being triggered.

javascript asynchronous async-await

Answer 1

Answer №1

When it comes to validateColor() and addColor(), they both generate Promise objects but are independent of each other. If you tweak addColor() to instead return the Promise from validateColor(), your code will function as intended:

    var colors = ["RED", "GREEN", "YELLOW"];
    
    const getColor = async () => {
      var value = "";
      colors.forEach((color) => {
        value = value + color + " ";
      });
      return value;
    };
    
    
    const middleware = async () => {
      addColor(null)
        .then(() => {
          getColor().then((result) => {
            console.log(result);
          });
        })
        .catch((err) => {
          console.log(err.message + " at  middleware");
        });
    };
    
    const addColor = async (color) => {
      return validateColor(color)
        .then(() => {
          console.log("Adding data");
          colors.push(color);
        })
        .catch((err) => {
          console.log(err.message + " at add color");
          throw err;
        });
    };
    
    const validateColor = async (color) => {
      if (color == null) {
        throw new Error("Color cannot be empty");
      }
    };
    
    middleware();

Your async functions wrap the return values in a Promise. Consequently, both validateColor() and addColor() will unknowingly create distinct Promises unless addressed. In contrast, by having addColor send back the same Promise that validateColor() generates, there is no additional separate Promise formed; the original Promise is passed back to middleware(). As this Promise contains the pending error thrown in validateColor(), the .catch() block will execute.

Answer 2

When it comes to validateColor() and addColor(), they both generate Promise objects but are independent of each other. If you tweak addColor() to instead return the Promise from validateColor(), your code will function as intended:

    var colors = ["RED", "GREEN", "YELLOW"];
    
    const getColor = async () => {
      var value = "";
      colors.forEach((color) => {
        value = value + color + " ";
      });
      return value;
    };
    
    
    const middleware = async () => {
      addColor(null)
        .then(() => {
          getColor().then((result) => {
            console.log(result);
          });
        })
        .catch((err) => {
          console.log(err.message + " at  middleware");
        });
    };
    
    const addColor = async (color) => {
      return validateColor(color)
        .then(() => {
          console.log("Adding data");
          colors.push(color);
        })
        .catch((err) => {
          console.log(err.message + " at add color");
          throw err;
        });
    };
    
    const validateColor = async (color) => {
      if (color == null) {
        throw new Error("Color cannot be empty");
      }
    };
    
    middleware();

Your async functions wrap the return values in a Promise. Consequently, both validateColor() and addColor() will unknowingly create distinct Promises unless addressed. In contrast, by having addColor send back the same Promise that validateColor() generates, there is no additional separate Promise formed; the original Promise is passed back to middleware(). As this Promise contains the pending error thrown in validateColor(), the .catch() block will execute.

Answer 3

Answer №2

If I had to sum it up in one sentence, I would say that every async function needs to include a return promise statement - here is the corrected code.

var colors = ["RED", "GREEN", "YELLOW"];

const getColor = async () => {
  var value = "";
  colors.forEach((color) => {
    value = value + color + " ";
  });
  return Promise.resolve(value);
};


const middleware = async () => {
  return addColor(null) 
    .then(() => {
      getColor().then((result) => {
        console.log(result);
      });
    })
    .catch((err) => {
      console.log(err.message + " at middleware");
    });
};

const addColor = async (color) => {
  return validateColor(color)
    .then(() => {
      console.log("Adding data");
      colors.push(color);
    })
    .catch((err) => {
      console.log(err.message + " at add color");
      throw err;
    });
};

const validateColor = async (color) => {
  if (color == null) {
    throw new Error("Color cannot be empty");
  }
  Promise.resolve(true);
};

middleware();

Explanation:

Simply put, an async function must return a promise in order for await statements to work effectively.

Example:

For instance, when calling a function as “test()” versus “await test()”, the former will execute and proceed with subsequent statements even during blocking operations, while the latter will wait for those operations to complete before moving on.

In this scenario, none of the async functions return promises by default, causing issues upon completion of their execution.

Detailed explanation specific to this example:

The initial call to “middleware()” triggers the chain of functions starting with “addColor()”.

addColor():
If the promise resolved from this function, it proceeds to call “getColor()”. Otherwise, it displays an error message.

Within addColor, we invoke validateColor() which performs the following check:

validateColor():
Upon resolving its promise, the color is added to the array; otherwise, an error is thrown.

Since validateColor encounters an error due to a null color, addColor erroneously assumes successful completion of its task.

Answer 4

If I had to sum it up in one sentence, I would say that every async function needs to include a return promise statement - here is the corrected code.

var colors = ["RED", "GREEN", "YELLOW"];

const getColor = async () => {
  var value = "";
  colors.forEach((color) => {
    value = value + color + " ";
  });
  return Promise.resolve(value);
};


const middleware = async () => {
  return addColor(null) 
    .then(() => {
      getColor().then((result) => {
        console.log(result);
      });
    })
    .catch((err) => {
      console.log(err.message + " at middleware");
    });
};

const addColor = async (color) => {
  return validateColor(color)
    .then(() => {
      console.log("Adding data");
      colors.push(color);
    })
    .catch((err) => {
      console.log(err.message + " at add color");
      throw err;
    });
};

const validateColor = async (color) => {
  if (color == null) {
    throw new Error("Color cannot be empty");
  }
  Promise.resolve(true);
};

middleware();

Explanation:

Simply put, an async function must return a promise in order for await statements to work effectively.

Example:

For instance, when calling a function as “test()” versus “await test()”, the former will execute and proceed with subsequent statements even during blocking operations, while the latter will wait for those operations to complete before moving on.

In this scenario, none of the async functions return promises by default, causing issues upon completion of their execution.

Detailed explanation specific to this example:

The initial call to “middleware()” triggers the chain of functions starting with “addColor()”.

addColor():
If the promise resolved from this function, it proceeds to call “getColor()”. Otherwise, it displays an error message.

Within addColor, we invoke validateColor() which performs the following check:

validateColor():
Upon resolving its promise, the color is added to the array; otherwise, an error is thrown.

Since validateColor encounters an error due to a null color, addColor erroneously assumes successful completion of its task.

Handling Errors in Asynchronous Functions with JavaScriptLet's explore the best practices for

Answer №1

Answer №2

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