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State updates are not applied immediately. Calling the setter schedules a re-render; it does not change the variable you are currently holding. Reading that variable on the next line gives you the
Generics let a function or type work with many types while preserving the relationship between what goes in and what comes out. Without them you face a choice between writing the same function
For describing the shape of an object they are largely interchangeable, and most codebases could use either consistently without noticing a difference. The distinctions matter at the edges. Type
When an event fires on an element, it does not stay there. It travels down from the document root to the target, then back up through every ancestor. Those two phases are called capturing and
The default sort converts every element to a string and compares those strings. So ten sorts before nine, because the character one comes before nine. The array is correctly sorted, just
Undefined means a value was never assigned. Null means a value was deliberately set to nothing. The language draws that distinction, and whether your code should is a style decision worth making
A shallow copy creates a new container holding references to the same inner objects. A deep copy recursively creates new copies of everything inside. The distinction only matters when the structure
Almost always because the whole file is being loaded into memory at once. Reading a file with a method that returns its entire contents, or building a list of every line before processing, requires
In development, Strict Mode deliberately renders components twice and runs effects twice, mounting, unmounting and remounting them. This does not happen in production builds. It is not a bug and not
The overwhelmingly common answer is the environment. Cron runs jobs with a deliberately minimal environment, quite unlike an interactive shell. Your profile scripts are not sourced, so the variables
Authentication establishes who someone is. Authorization determines what that identity is allowed to do. They are sequential: you cannot meaningfully decide permissions before you know the identity,
High memory in a long-running process is either legitimate caching, a genuine leak, or garbage collection that has not needed to run yet. Distinguishing them is the first task, because the response
Start with the part people skip: if a credential was pushed anywhere others could read it, treat it as compromised and rotate it first. Rewriting history removes the file from the repository, but it
A unit test exercises one piece of logic in isolation, replacing its collaborators with stand-ins. An integration test exercises several real components together, typically including something
The rule is simple to state: use a transaction whenever two or more writes must either all succeed or all fail. If a partial result would leave your data in a state that should never exist, those
The compiler is telling you that a value type includes undefined and you are using it as though it does not. With strict null checking enabled, undefined and null are not silently assignable to other
An image is a static, read-only template. A container is a running, or stopped, instance created from one. The relationship is close to that between a program on disk and a process running it: one
The message has three distinct causes that look nearly identical, and knowing which one you have saves a great deal of guessing. The first and most common is that the file is not marked executable.
An operation is idempotent when performing it more than once has the same effect as performing it once. Setting a value to ten is idempotent; adding ten is not. The property matters because networks
A 502 is reported by a proxy or load balancer, not by your application. It means the intermediary tried to forward the request to an upstream server and received an invalid response, or none at all.
The gap is almost always something present on your machine and absent from the runner. Continuous integration starts from a clean environment every time, which is the point, and that clean start
The short version is that 401 concerns identity and 403 concerns permission. A 401 means the request was not authenticated: no credentials were supplied, or the ones supplied were not valid. A 403
Pool exhaustion means connections are being taken out and not returned as fast as they are requested. The symptom is a request that hangs and then times out waiting to acquire one, and it typically
An inner join returns only rows where the condition matched on both sides. A left join returns every row from the left table regardless, filling the right side with nulls where nothing matched. That