karaoke ni ikou.

に marks the destination of movement here.

In カラオケ に 行こう, the verb 行く means to go, and に shows where you are going:

• 学校に行く = go to school
• 駅に行く = go to the station
• カラオケに行こう = let’s go to karaoke

With motion verbs like 行く, に often works like to in English.

行こう is the volitional form of 行く.

The volitional form is often used to mean:

• let’s go
• I’ll go
• shall we go? depending on context

So:

• 行く = go / will go
• 行こう = let’s go

In this sentence, 行こう sounds like a suggestion or invitation.

Yes. 行こう is the plain/casual form.

A more polite version would be:

• カラオケに行きましょう。

Both can mean Let’s go to karaoke, but the tone is different:

• 行こう = casual, used with friends, family, people you are close to
• 行きましょう = polite, softer and more formal

The subject is omitted, which is very normal in Japanese.

In English, you usually need to say something like:

• Let’s go to karaoke
• Shall we go to karaoke?

In Japanese, the who is often understood from context. With 行こう, the implied subject is usually:

• I and you
• or we

So the sentence naturally means something like Let’s go to karaoke.

カラオケ is conventionally written in katakana.

Katakana is often used for:

• loanwords
• foreign-sounding words
• modern commercial terms
• emphasis/stylistic effect

Even though karaoke is a Japanese word historically, it is normally written as カラオケ, not in hiragana in everyday writing.

It can mean either, depending on context, but in this sentence it most naturally refers to going to a karaoke place / karaoke room / karaoke bar.

Because the verb is 行く (to go), Japanese listeners will usually understand:

• Let’s go to karaoke
• meaning Let’s go to a place where we can do karaoke

So カラオケに行く is like saying go to karaoke in natural English.

Yes, you can say:

• カラオケへ行こう。

Both に and へ can mark destination with 行く.

The difference is small here:

• に focuses more directly on the destination
• へ can feel a little more like toward

In everyday conversation, カラオケに行こう is very common and natural.

Because 行く does not take カラオケ as a direct object here.

を marks a direct object, but with 行く, the place you go to is normally marked by:

• に
• or へ

So:

• カラオケに行く = go to karaoke

Using を here would be incorrect in normal Japanese.

It is usually an invitation or suggestion.

行こう often means:

• Let’s go
• Come on, let’s go

Depending on tone and situation, it can sound:

• enthusiastic
• casual
• encouraging

It is not usually a command like Go!
A command would use a different form.

Those spaces are usually added only for learners.

Normal Japanese writing would be:

• カラオケに行こう。

Japanese normally does not use spaces between words the way English does. Textbooks and beginner materials sometimes add spaces to make the sentence easier to read.