Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. In TypeScript, I can declare a parameter of a function as a type Function. Is there a "type-safe" way of doing this that I am missing? For example, consider this:. The save callback is not type safe, I am giving it a callback function where the function's parameter is a string but I am passing it a number, and compiles with no errors.
Can I make the result parameter in save a type-safe function?What does grossly unremarkable mean
A function's type consists of the types of its argument and its return type. Here we specify that the callback parameter's type must be "function that accepts a number and returns type any ":. If you want, you can define a type alias to encapsulate this:.
I realize this post is old, but there's a more compact approach that is slightly different than what was asked, but may be a very helpful alternative. You can essentially declare the function in-line when calling the method Foo 's save in this case. It would look something like this:. The multipleCallback approach is very useful for things like network calls that may succeed or fail. Again assuming a network call example, when multipleCallbacks is called, behavior for both a success and failure can be defined in one spot, which lends itself to greater clarity for future code readers.
Generally, in my experience, this approach lends itself to being more concise, less clutter, and greater clarity overall. Function type literals are another way to declare the type of a function. They're usually applied in the function signature of a higher-order function. A higher-order function is a function which accepts functions as parameters or which returns a function.
It has the following syntax:. Besides what other said, a common problem is to declare the types of the same function that is overloaded. Typical case is EventEmitter on method which will accept multiple kind of listeners.
Similar could happen When working with redux actions - and there you use the action type as literal to mark the overloading, In case of EventEmitters, you use the event name literal type:.
Because you can't easily union a function definition and another data type, I find having these types around useful to strongly type them.Voyage mom jeans alla caviglia a vita medio alta wow blue ldxu0hij
Based on Drew's answer. Now you can strongly type every parameter and the return type! Here's an example with more parameters than what is above.
Now you can write a union type, like an object or a function returning an object, without creating a brand new type that may need to be exported or consumed. Learn more.
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We can add types to each of the parameters and then to the function itself to add a return type. TypeScript can figure the return type out by looking at the return statements, so we can also optionally leave this off in many cases.
When writing out the whole function type, both parts are required. We write out the parameter types just like a parameter list, giving each parameter a name and a type.
This name is just to help with readability. We could have instead written:. The second part is the return type. Of note, only the parameters and the return type make up the function type. Captured variables are not reflected in the type. In playing with the example, you may notice that the TypeScript compiler can figure out the type even if you only have types on one side of the equation:. This helps cut down on the amount of effort to keep your program typed.
In TypeScript, every parameter is assumed to be required by the function. The compiler also assumes that these parameters are the only parameters that will be passed to the function.Aarti khurana quotes
In short, the number of arguments given to a function has to match the number of parameters the function expects.
In TypeScript, we can also set a value that a parameter will be assigned if the user does not provide one, or if the user passes undefined in its place. These are called default-initialized parameters.
Default-initialized parameters that come after all required parameters are treated as optional, and just like optional parameters, can be omitted when calling their respective function. This means optional parameters and trailing default parameters will share commonality in their types, so both.
The default value of lastName disappears in the type, only leaving behind the fact that the parameter is optional. If a default-initialized parameter comes before a required parameter, users need to explicitly pass undefined to get the default initialized value.
For example, we could write our last example with only a default initializer on firstName :. Required, optional, and default parameters all have one thing in common: they talk about one parameter at a time. Sometimes, you want to work with multiple parameters as a group, or you may not know how many parameters a function will ultimately take.
We will start with a very popular example where we will call an already defined function with the onClick event. We will just provide the function name to the onClick event. This code can be a single statement or multiple statements. In this example, we will run the alert function and make some calculations. If we will use multiple statements in a onClick event we have to delimit the statements with. But if there is a single statement ; is optional where we can use or not. We can also provide some parameters to the function which is called with the onClick event.
We will just call the function and put the parameters inside the brackets. Up to now, we have used a simple element attribute way for the onClick event.
We will set the elements DOM object onclick attribute. We will just set the onclick attribute of the element to the null object which will remove the existing assignment. We will just get the object of the HTML element and use addEventListener function with the event name and the code we can to assign.
The onClick event is named as click for the addEventListerner. We can also use the addEvenListener function in order to remove existing click event code or function. We will just empty string for code or function parameter like below.
In fact, it is invalid since the second parameter must be a function.A common thing you will want to learn in React is how to pass a value as a parameter through the onClick event handler. Read on to learn how! To understand how to pass a value as a parameter through an onClick event handler, take a look at the line of code inside of the return statement.
Typically, to call a function when we click a button in React, we would simply pass in the name of the function to the onClick handler, like so:. In order to pass a value as a parameter through the onClick handler we pass in an arrow function which returns a call to the sayHello function. You might want to pass in the value or name of the button or input element through the event handler. The button now has an additional attribute, named value. The example above shows the variable e that is provided from the onClick event handler.
This stands for event. Once we have the event, we can access values such as the value or name attribute. Your email address will not be published. Calling a function without a parameter. Don't miss the latest React tutorials. No spam, ever. Opt out at any time. If you'd like to receive a friendly email once in a while of all new React tutorials, just pop your email above!
I appreciate the support! Email address We will never share your email with anyone else. Comments Kumar says:.
December 26, at pm. James King says:. January 3, at pm.Example 1: This example simply put the argument which is string in the onClick attribute of the button which calls a function with a string as an argument using onClick method.
Example 2: This example simply put the argument which is a string in the onClick attribute of the button which calls a function with string as an argument using onClick method. Here the input button is created dynamically. This example uses the same approach as the previous one. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.
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Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I'm trying to get rid of my tslint error Type declaration of 'any' loses type-safety.
I have tried to declare the event as a few different types, like React. I can see from the inspector that the currentTarget is Konva. Text and the index is set to 0 but not sure that helps me as I can't set the type to Konva. Textwhich would make sense to me, but that doesn't work either. I think the index is being added by the Konva Layer class but I'm pretty new to the whole react ecosystem so still trying to wrap my brain around it all.
I was able use declaration merging suggestion by Tyler Sebastion to define the index on the target which silenced tslint. I'm not sure this is the best approach though as it feels a bit fragile to me. As posted in my update above, a potential solution would be to use Declaration Merging as suggested by Tyler-sebastion. I was able to define two additional interfaces and add the index property on the EventTarget in this way. You should be using event. React is mirroring the difference between currentTarget element the event is attached to and target the element the event is currently happening on.
Specifying onClick event type with Typescript and React. Konva Ask Question. Asked 3 years, 2 months ago. Active 6 months ago. Viewed 86k times. Justin Levi Winter. Active Oldest Votes. Tyler Sebastian Tyler Sebastian 6, 4 4 gold badges 32 32 silver badges 55 55 bronze badges. Thanks for the idea. I was going to try and cast the target but never got around to trying. I'll definitely give that a shot although that definitely feels a bit of a workaround. I will definitely read up on the declaration merging as I haven't seen that before.
Aug 10 '18 at I copied from OP's code. This could trivially be moved into a React. Asimov Asimov 5 5 silver badges 14 14 bronze badges. Taken from the ReactKonvaCore. Roland Zwaga Roland Zwaga 21 1 1 bronze badge.
When I click on this button, I have an error that says that "Add is not defined". Since this function call works perfectly with a numeric parameter, I assume that it has something to do with the symbols "" in the string. It looks like you're building DOM elements from strings. You just need to add some quotes around result. Just be aware that if this is a loop or something, result will change before the event fires and you'd need to create an additional scope bubble to shadow the changing variable.
Couple of concerns for me with respect to using string escape in onClick and as the number of arguments grow, it will become cumbersome to maintain. The following approach will have a one hop - On click - take the control to a handler method and handler method, based on the event object, can deduct the click event and corresponding object. The advantage here is that we can have as many arguments in above example, data-arg1, data-arg I suggest not even using HTML onclick handlers, and use something more common such as document.
You can pass refrence or string value just put function inside the doube commas "" asp below snapshot. In most of the cases it works with every browser. If you are adding button or link dynamically and facing the issue then this may be help. I solved by this way. So may be my code will not be optimized or syntax, but it was working for me. In Razoryou can pass parameters dynamically:. If you are using ASP.Thomas kavaler
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